docs: Fix GitHub PR links (#3346)

Fix the `gh-pr` special markup to use the right URL.

It was using `/pulls/` which is a search, when it should be `/pull/`

docs/conf.py

diff --git a/docs/conf.py b/docs/conf.py
index 47ab378..671cf23 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -123,7 +123,7 @@
 extlinks = {
     "gh-issue": (f"https://github.com/bazel-contrib/rules_python/issues/%s", "#%s issue"),
     "gh-path": (f"https://github.com/bazel-contrib/rules_python/tree/main/%s", "%s"),
-    "gh-pr": (f"https://github.com/bazel-contrib/rules_python/pulls/%s", "#%s PR"),
+    "gh-pr": (f"https://github.com/bazel-contrib/rules_python/pull/%s", "#%s PR"),
 }
 
 # --- MyST configuration