Updated the default class name

• Updated:
- the default class name

app/javascript/gabsocial/components/panel/sign_up_panel.js

@@ -31,7 +31,7 @@ class SignUpPanel extends React.PureComponent {
         title={intl.formatMessage(messages.title)}
         subtitle={intl.formatMessage(messages.subtitle)}
       >
-        <div className={[_s._, _s.flexRow, _s.pb5].join(' ')}>
+        <div className={[_s.d, _s.flexRow, _s.pb5].join(' ')}>
           <Button
             isOutline
             color='brand'