test: fix protocol-relative URLs in newsletters

Change `//github.com/` to `https://github.com/` to ensure
links work correctly and pass html-proofer validation.

Author: stickies-v

_posts/en/newsletters/2018-11-06-newsletter.md

@@ -154,7 +154,7 @@ repo].*
 [bech32 patch]: https://github.com/sipa/bech32/commit/2b0aac650ce560fb2b2a2bebeacaa5c87d7e5938
 [Bitcoin Core 0.17.0.1]: https://bitcoincore.org/en/releases/0.17.0.1/
 [bech32 overflow blog]: https://blog.trezor.io/details-about-the-security-updates-in-trezor-one-firmware-1-7-1-5c34278425d8
-[bech32 refs]: //github.com/sipa/bech32/tree/master/ref/
+[bech32 refs]: https://github.com/sipa/bech32/tree/master/ref/
 [trezor-crypto]: https://github.com/trezor/trezor-crypto/
 [cashaddr patch]: https://github.com/trezor/trezor-crypto/commit/2bbbc3e15573294c6dd0273d2a8542ba42507eb0
 [ln bolt11 ss]: https://gnusha.org/url/https://lists.linuxfoundation.org/pipermail/lightning-dev/2018-November/001489.html

_posts/en/newsletters/2020-04-08-newsletter.md

@@ -257,7 +257,7 @@ hidden service if enabled at [the command line][watchtower tor].
 [uSEkaCIO email]: https://gnusha.org/url/https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-November/002316.html
 [jonasnick otves]: https://github.com/jonasnick/secp256k1/pull/14/
 [nkohen otves]: https://github.com/bitcoin-s/bitcoin-s/pull/1302
-[lnd psbt]: //github.com/lightningnetwork/lnd/blob/master/docs/psbt.md
+[lnd psbt]: https://github.com/lightningnetwork/lnd/blob/master/docs/psbt.md
 [review club 18238]: https://bitcoincore.reviews/18238.html
 [wtxid transaction relay]: https://bitcoincore.reviews/18044
 [watchtower tor]: https://github.com/lightningnetwork/lnd/blob/master/docs/watchtower.md#tor-hidden-services

_posts/zh/newsletters/2018-11-06-newsletter.md

@@ -75,7 +75,7 @@ lang: zh
 [bech32 patch]: https://github.com/sipa/bech32/commit/2b0aac650ce560fb2b2a2bebeacaa5c87d7e5938
 [Bitcoin Core 0.17.0.1]: https://bitcoincore.org/en/releases/0.17.0.1/
 [bech32 overflow blog]: https://blog.trezor.io/details-about-the-security-updates-in-trezor-one-firmware-1-7-1-5c34278425d8
-[bech32 refs]: //github.com/sipa/bech32/tree/master/ref/
+[bech32 refs]: https://github.com/sipa/bech32/tree/master/ref/
 [trezor-crypto]: https://github.com/trezor/trezor-crypto/
 [cashaddr patch]: https://github.com/trezor/trezor-crypto/commit/2bbbc3e15573294c6dd0273d2a8542ba42507eb0
 [ln bolt11 ss]: https://gnusha.org/url/https://lists.linuxfoundation.org/pipermail/lightning-dev/2018-November/001489.html

_posts/zh/newsletters/2020-04-08-newsletter.md

@@ -121,7 +121,7 @@ _在本节中，我们总结了一次最近的 Bitcoin Core PR 审查俱乐部
 [uSEkaCIO email]: https://gnusha.org/url/https://lists.linuxfoundation.org/pipermail/lightning-dev/2019-November/002316.html
 [jonasnick otves]: https://github.com/jonasnick/secp256k1/pull/14/
 [nkohen otves]: https://github.com/bitcoin-s/bitcoin-s/pull/1302
-[lnd psbt]: //github.com/lightningnetwork/lnd/blob/master/docs/psbt.md
+[lnd psbt]: https://github.com/lightningnetwork/lnd/blob/master/docs/psbt.md
 [review club 18238]: https://bitcoincore.reviews/18238.html
 [wtxid transaction relay]: https://bitcoincore.reviews/18044
 [watchtower tor]: https://github.com/lightningnetwork/lnd/blob/master/docs/watchtower.md#tor-hidden-services