Correct name is login_user

Fixes #1848.

library/mysql_db

@@ -73,7 +73,7 @@ examples:
 notes:
    - Requires the MySQLdb Python package on the remote host. For Ubuntu, this
      is as easy as apt-get install python-mysqldb. (See M(apt).)
-   - Both I(login_password) and I(login_username) are required when you are
+   - Both I(login_password) and I(login_user) are required when you are
      passing credentials. If none are present, the module will attempt to read
      the credentials from C(~/.my.cnf), and finally fall back to using the MySQL
      default login of C(root) with no password.