Fixed Eslint validation.

web/react/components/signup_user_complete.jsx

@@ -154,7 +154,7 @@ class SignupUserComplete extends React.Component {
             <span className='help-block'>
                 <FormattedMessage
                     id='signup_user_completed.emailHelp'
-                    defaultMessage="Valid email required for sign-up"
+                    defaultMessage='Valid email required for sign-up'
                 />
             </span>
         );