Added anchor for permissions

| Q   | A
| --- | ---
| Doc fix? | yes
| New docs? | no
| Applies to | all
| Fixed tickets | -

This is quite a common problem on IRC. #symfony IRC now has a nice bot which parses the docs and indexes it, so we can easily link to sections (thanks to the great @mitom). If a sidebar doesn't have an anchor, it's impossible to link to it correctly. Adding a named anchor fixes that.