Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
or edited some other way to fix the grammar.

Lib/concurrent/futures/_base.py

@@ -518,7 +518,7 @@ class Executor(object):
         raise NotImplementedError()
 
     def map(self, fn, *iterables, timeout=None):
-        """Returns a iterator equivalent to map(fn, iter).
+        """Returns an iterator equivalent to map(fn, iter).
 
         Args:
             fn: A callable that will take as many arguments as there are