Merge pull request #2673 from jryans/username-label

Clarify what the username error refers to

src/components/structures/auth/Registration.js

@@ -313,7 +313,7 @@ module.exports = React.createClass({
                 errMsg = _t('A phone number is required to register on this homeserver.');
                 break;
             case "RegistrationForm.ERR_USERNAME_INVALID":
-                errMsg = _t("Only use lower case letters, numbers and '=_-./'");
+                errMsg = _t("A username can only contain lower case letters, numbers and '=_-./'");
                 break;
             case "RegistrationForm.ERR_USERNAME_BLANK":
                 errMsg = _t('You need to enter a username.');