Fix lint: use unescaped braces in regex (unicorn/better-regex)

Author: johanrd

lib/rules/template-no-mut-helper.js

@@ -36,7 +36,7 @@ module.exports = {
     // don't end up with `{{{{...}}}}` in the error message.
     const setterAlternative =
       typeof rawSetterAlternative === 'string'
-        ? rawSetterAlternative.replace(/^\{\{([\S\s]*)\}\}$/, '$1').trim()
+        ? rawSetterAlternative.replace(/^{{([\S\s]*)}}$/, '$1').trim()
         : rawSetterAlternative;
     const message = setterAlternative
       ? `Do not use the (mut) helper. Consider using a JS action or {{${setterAlternative}}} instead.`