rename "django_htmx" templatetag to "htmx"

[nerdoc]

Description

It's just a small change, and would even break compatibility...
But I dare to suggest renaming it to "htmx" - just because {% load htmx %} sounds clearer.
There is already a djhtmx library which hast this tag - but anyway, both won't be installed together anyway.

[adamchainz]

I would rather be more explicit, and not cause churn.