[Doc] Add link to tool_choice tracking issue in tool_calling.md (#13003)

Signed-off-by: Yuan Tang <terrytangyuan@gmail.com>

docs/source/features/tool_calling.md

 # Tool Calling
 
-vLLM currently supports named function calling, as well as the `auto` and `none` options for the `tool_choice` field in the chat completion API. The `tool_choice` option `required` is **not yet supported** but on the roadmap.
+vLLM currently supports named function calling, as well as the `auto` and `none` options for the `tool_choice` field in the chat completion API. The `tool_choice` option `required` is **not yet supported** but [on the roadmap](gh-issue:13002).
 
 ## Quickstart