fix typo

lib/models/external-account.js

@@ -89,11 +89,11 @@ class ExternalAccount {
             // when the User that have the same `username` exists
             if (user != null) {
               if (isTreatUsernameMatchingAsIdentical) {
-                debug(`ExternalAccount '${accountId}' will be bind to an exisiting account`);
+                debug(`ExternalAccount '${accountId}' will be bound to an exisiting account`);
                 return user;
               }
               else {
-                throw new DuplicatedUsernameException(`Userr '${usernameToBeRegistered}' has already been existed`);
+                throw new DuplicatedUsernameException(`User '${usernameToBeRegistered}' has already been existed`);
               }
             }