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.

docs/ref/models/expressions.txt

@@ -331,7 +331,7 @@ 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):
                 ...

docs/ref/models/fields.txt

@@ -1458,7 +1458,7 @@ Relationships defined this way on :ref:`abstract models
 concrete model and are not relative to the abstract model's ``app_label``:
 
 .. code-block:: python
-    :caption: products/models.py
+    :caption: ``products/models.py``
 
     from django.db import models
 
@@ -1469,7 +1469,7 @@ concrete model and are not relative to the abstract model's ``app_label``:
             abstract = True
 
 .. code-block:: python
-    :caption: production/models.py
+    :caption: ``production/models.py``
 
     from django.db import models
     from products.models import AbstractCar