Fix link

Author: colesbury

Doc/whatsnew/3.15.rst

@@ -216,7 +216,7 @@ Porting to Python 3.15
 
   * ``PyMutex_IsLocked()`` : :c:func:`PyMutex_IsLocked`
 
-  The `pythoncapi-compat project`_ can be used to get most of these new
+  The |pythoncapi_compat_project| can be used to get most of these new
   functions on Python 3.14 and older.
 
 Deprecated C APIs