remove unnecessary class

src/client/js/components/PageEditor/CodeMirrorEditor.jsx

@@ -549,7 +549,7 @@ export default class CodeMirrorEditor extends AbstractEditor {
 
   renderCheatsheetModalButton() {
     return (
-      <button type="button" className="btn-link gfm-cheatsheet-modal-link text-muted small p-0" onClick={() => { this.markdownHelpButtonClickedHandler() }}>
+      <button type="button" className="btn-link gfm-cheatsheet-modal-link small p-0" onClick={() => { this.markdownHelpButtonClickedHandler() }}>
         <i className="icon-question" /> Markdown
       </button>
     );