[4.0.x] Fixed docs build with sphinxcontrib-spelling 7.5.0+.

sphinxcontrib-spelling 7.5.0+ includes captions of figures in the set
of nodes for which the text is checked.

Backport of ac90529cc5 from main.

docs/ref/models/expressions.txt

@@ -335,7 +335,7 @@ The ``Func`` API is as follows:
         customize the SQL as needed. For example:
 
         .. code-block:: python
-            :caption: django/db/models/functions.py
+            :caption: ``django/db/models/functions.py``
 
             class ConcatPair(Func):
                 ...