Fix documentation on using prettier for a specific directory

If the `check` parameter is used, the `allFiles` parameter is `false`,
which causes the prettier.js script only to process staged files.
The correct parameters are `check-all` and `save-all`.

doc/development/new_fe_guide/style/prettier.md

@@ -47,13 +47,13 @@ The source of these Yarn scripts can be found in `/scripts/frontend/prettier.js`
 ### Scripts during Conversion period
 
 ```
-node ./scripts/frontend/prettier.js check ./vendor/
+node ./scripts/frontend/prettier.js check-all ./vendor/
 ```
 
 This will go over all files in a specific folder check it.
 
 ```
-node ./scripts/frontend/prettier.js save ./vendor/
+node ./scripts/frontend/prettier.js save-all ./vendor/
 ```
 
 This will go over all files in a specific folder and save it.