Clarify what the username error refers to

Fixes https://github.com/vector-im/riot-web/issues/8839

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.');

src/components/views/dialogs/SetMxIdDialog.js

@@ -115,7 +115,7 @@ export default React.createClass({
         // user ID roughly looks okay from a Matrix perspective.
         if (!SAFE_LOCALPART_REGEX.test(this.state.username)) {
             this.setState({
-                usernameError: _t("Only use lower case letters, numbers and '=_-./'"),
+                usernameError: _t("A username can only contain lower case letters, numbers and '=_-./'"),
             });
             return Promise.reject();
         }

src/i18n/strings/en_EN.json

@@ -1132,7 +1132,7 @@
     "Email address": "Email address",
     "This will allow you to reset your password and receive notifications.": "This will allow you to reset your password and receive notifications.",
     "Skip": "Skip",
-    "Only use lower case letters, numbers and '=_-./'": "Only use lower case letters, numbers and '=_-./'",
+    "A username can only contain lower case letters, numbers and '=_-./'": "A username can only contain lower case letters, numbers and '=_-./'",
     "Username not available": "Username not available",
     "Username invalid: %(errMessage)s": "Username invalid: %(errMessage)s",
     "An error occurred: %(error_string)s": "An error occurred: %(error_string)s",