Fix bpo-39416: Change "Numeric" to lower case; an english word, not a…

… class name (pythonGH-22867)

This is a trivial fix to [bpo-39416](), which didn't come up until it was already committed

```
Change "Numeric" to "numeric".

I believe this is trivial enough to not need an issue or a NEWS entry, although
I'm unclear on what branches the original pull request received backports.
```

Automerge-Triggered-By: GH:merwok

Doc/reference/datamodel.rst

@@ -187,7 +187,7 @@ Ellipsis
    related to mathematical numbers, but subject to the limitations of numerical
    representation in computers.
 
-   The string representations of the Numeric classes, computed by
+   The string representations of the numeric classes, computed by
    :meth:`__repr__` and :meth:`__str__`, have the following
    properties: