fetch: fix the flat parameter description

ansible-doc expects the value of the description field to be a list,
otherwise the output is not correct. This patch updates the flat
description to be a list.

lib/ansible/modules/files/fetch.py

@@ -45,10 +45,10 @@ options:
   flat:
     version_added: "1.2"
     description:
-      Allows you to override the default behavior of prepending hostname/path/to/file to
+      - Allows you to override the default behavior of prepending
-      the destination.  If dest ends with '/', it will use the basename of the source
+        hostname/path/to/file to the destination.  If dest ends with '/', it
-      file, similar to the copy module.  Obviously this is only handy if the filenames
+        will use the basename of the source file, similar to the copy module.
-      are unique.
+        Obviously this is only handy if the filenames are unique.
 requirements: []
 author: Michael DeHaan
 '''