Fix typo in config options example

There is no `BABEL_DOMAINS`, is just `BABEL_DOMAIN`. I introduced this back in  python-babel#194

Author: amercader

docs/index.rst

@@ -57,7 +57,7 @@ to change some internal defaults:
                                 translation directories, eg::
 
                                     BABEL_TRANSLATION_DIRECTORIES=/path/to/translations;/another/path/
-                                    BABEL_DOMAINS=messages;myapp
+                                    BABEL_DOMAIN=messages;myapp
 
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