[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/internals/contributing/writing-code/coding-style.txt

@@ -171,7 +171,7 @@ Imports
   For example (comments are for explanatory purposes only):
 
   .. code-block:: python
-      :caption: django/contrib/admin/example.py
+      :caption: ``django/contrib/admin/example.py``
 
       # future
       from __future__ import unicode_literals

docs/internals/contributing/writing-code/unit-tests.txt

@@ -563,7 +563,7 @@ Since this pattern involves a lot of boilerplate, Django provides the
     installed, you should pass the set of targeted ``app_label`` as arguments:
 
     .. code-block:: python
-        :caption: tests/app_label/tests.py
+        :caption: ``tests/app_label/tests.py``
 
         from django.db import models
         from django.test import SimpleTestCase