Clean-up example of using fileinput as a context manager.

Doc/library/fileinput.rst

@@ -58,8 +58,9 @@ The following function is the primary interface of this module:
    :keyword:`with` statement.  In this example, *input* is closed after the
    :keyword:`with` statement is exited, even if an exception occurs::
 
-      with fileinput.input(files=('spam.txt', 'eggs.txt')) as input:
-          process(input)
+      with fileinput.input(files=('spam.txt', 'eggs.txt')) as f:
+          for line in f:
+              process(line)
 
    .. versionchanged:: 3.2
       Can be used as a context manager.