bpo-42294: Grammar fixes in doc glossary strong/weak refs (GH-23227)

diff --git a/Doc/c-api/weakref.rst b/Doc/c-api/weakref.rst
index fb6628a..9205ca6 100644
--- a/Doc/c-api/weakref.rst
+++ b/Doc/c-api/weakref.rst
@@ -59,7 +59,7 @@
 
       This function returns a :term:`borrowed reference` to the referenced object.
       This means that you should always call :c:func:`Py_INCREF` on the object
-      except it cannot be destroyed before the last usage of the borrowed
+      except when it cannot be destroyed before the last usage of the borrowed
       reference.