Update physics.rst

[ramon-adan]

Add division symbol macro to the description

[dpvc]

The symbol lists are generated programmatically from the package files themselves, so we don't make hand changes to these lists. The reason \divsymbol is not listed here is that it is in the base package (though that should probably be changed). Since the LaTeX physics package uses \divisionsymbol while physics2 uses \divsymbol, we should probably include both.

In any case, because these are auto-generated, I'm closing this PR.