docs: fix vim.treesitter tags

Problem:
Help tags like vim.treesitter.language.add() are confusing because
`vim.treesitter.language` is (thankfully) not a user-facing module.

Solution:
Ignore the "fstem" when generating "treesitter" tags.

scripts/gen_vimdoc.py

@@ -286,7 +286,7 @@ CONFIG = {
             if fstem == 'treesitter'
             else f'*{name}()*'
             if name[0].isupper()
-            else f'*vim.treesitter.{fstem}.{name}()*'),
+            else f'*vim.treesitter.{name}()*'),
         'module_override': {},
         'append_only': [],
     }