Update i18n.md (#1011)

Fix the English locale link

docs/i18n.md

@@ -74,7 +74,7 @@ use this quick guide:
 - First of all, [create an issue](https://github.com/date-fns/date-fns/issues/new?title=XXX%20language%20support)
   so you won't overlap with others.
 - A detailed explanation of how to [add a new locale](https://github.com/date-fns/date-fns/blob/master/docs/i18nContributionGuide.md#adding-a-new-locale).
-- Use [English locale](https://github.com/date-fns/date-fns/tree/master/src/locale/en)
+- Use [English locale](https://github.com/date-fns/date-fns/tree/master/src/locale/en-US)
   as the basis and then incrementally adjust the tests and the code.
 - Directions on [adding a locale with the same language as another locale](https://github.com/date-fns/date-fns/blob/master/docs/i18nContributionGuide.md#creating-a-locale-with-the-same-language-as-another-locale).
 - If you have questions or need guidance, leave a comment in the issue.