Update extlinks strings to use a format string (#555)

This addresses a warning in Sphinx 5, regarding the missing` %s` in
these strings.

```
extlinks: Sphinx-6.0 will require a caption string to contain exactly one '%s' and all other '%' need to be escaped as '%%'.
```

docs/conf.py

@@ -52,8 +52,8 @@
 exclude_patterns = ["_build"]
 
 extlinks = {
-    "issue": ("https://github.com/pypa/packaging/issues/%s", "#"),
-    "pull": ("https://github.com/pypa/packaging/pull/%s", "PR #"),
+    "issue": ("https://github.com/pypa/packaging/issues/%s", "#%s"),
+    "pull": ("https://github.com/pypa/packaging/pull/%s", "PR #%s"),
 }
 # -- Options for HTML output --------------------------------------------------