minor #15262 update doc with max-age (trollepierre)

This PR was submitted for the 5.2 branch but it was merged into the 4.4 branch instead.

Discussion
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update doc with max-age

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Commits
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d12df22 update doc with max-age

Author: javiereguiluz

http_cache/expiration.rst

@@ -34,7 +34,7 @@ additional directives):
 
 .. code-block:: text
 
-    Cache-Control: public, maxage=600
+    Cache-Control: public, max-age=600
 
 .. note::