refactor: replace deprecated String.prototype.substr()

.substr() is deprecated so we replace it with .slice() which works similarily but isn't deprecated

Signed-off-by: Tobias Speicher <[email protected]>

Author: CommanderRoot

packages/graphql-playground-react/config/paths.js

@@ -14,7 +14,7 @@ const envPublicUrl = process.env.PUBLIC_URL
 function ensureSlash(path, needsSlash) {
   const hasSlash = path.endsWith('/')
   if (hasSlash && !needsSlash) {
-    return path.substr(path, path.length - 1)
+    return path.slice(0, -1)
   } else if (!hasSlash && needsSlash) {
     return `${path}/`
   } else {