l10n: README: fix typo

This 10-year old typo was introduced at 75b182ae (Update l10n guide:
change the repository URL, etc, 2012-03-02). The word "l10" should be
"l10n".

Signed-off-by: Arthur Milchior <arthur@milchior.fr>
Signed-off-by: Jiang Xin <worldhello.net@gmail.com>

po/README.md

@@ -5,7 +5,7 @@ describes how you can contribute to the effort of enhancing the language
 coverage and maintaining the translation.
 
 The localization (l10n) coordinator, Jiang Xin <worldhello.net@gmail.com>,
-coordinates our localization effort in the l10 coordinator repository:
+coordinates our localization effort in the l10n coordinator repository:
 
     https://github.com/git-l10n/git-po/