Fix missing punctuation in concurrent.interpreters docs

Add missing periods to a sentence in the module introduction. The
sentence about threading was missing its ending period after the
:mod: reference and at the end of the sentence.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Anandesh-Sharma

Doc/library/concurrent.interpreters.rst

@@ -21,7 +21,7 @@ thread) and calling a function in that execution context.
 For concurrency, interpreters themselves (and this module) don't
 provide much more than isolation, which on its own isn't useful.
 Actual concurrency is available separately through
-:mod:`threads <threading>`  See `below <interp-concurrency_>`_
+:mod:`threads <threading>`.  See `below <interp-concurrency_>`_.
 
 .. seealso::