docs(luacats): support backtick captured generic type

Problem:
While LuaCATS's generics system are still considered WIP by luals, they
currently support type captured generics.

See "Capture with Backtick" example:
https://luals.github.io/wiki/annotations/#generic

Solution:
Add support for it in the LuaCATS grammar

test/functional/script/luacats_grammar_spec.lua

@@ -152,4 +152,11 @@ describe('luacats grammar', function()
     name = '[1]',
     type = 'integer',
   })
+
+  test('@param type `T` this is a generic type', {
+    desc = 'this is a generic type',
+    kind = 'param',
+    name = 'type',
+    type = '`T`',
+  })
 end)