Fix indentation in 'see also'.

The only necessary tweak was deleting one space from the line with the comment '.. XXX Add references...' because the four spaces were making the whole thing be interpreted as a blockquote.
See more: https://stackoverflow.com/a/4783854

Author: rodrigogiraoserrao

Doc/library/concurrent.interpreters.rst

@@ -29,12 +29,12 @@ Actual concurrency is available separately through
 .. seealso::
 
    :class:`~concurrent.futures.InterpreterPoolExecutor`
-      combines threads with interpreters in a familiar interface.
+      Combines threads with interpreters in a familiar interface.
 
-    .. XXX Add references to the upcoming HOWTO docs in the seealso block.
+   .. XXX Add references to the upcoming HOWTO docs in the seealso block.
 
    :ref:`isolating-extensions-howto`
-       how to update an extension module to support multiple interpreters
+      How to update an extension module to support multiple interpreters.
 
    :pep:`554`