Fix a name for `backtick_tag_function`

[acashjos]

Checkout 8ef4017

Remember backtick_tag_function is not the name of function.
It is backTickTagFn, this doesn't need to be changed.
What is to be changed is the global variable name when this fn is injected into template. Currently there are 3 variants namely bt, bt_ and bt__. One must be retained and others removed