Rename remaining mentions of gtkcellrendererexpander into pidgincellrendererexpander

After switching from gtkcellrendererexpander to pidgincellrendererexpander there were a few remaining things to rename, one of which was the docs reference file which broke the docs build

Testing Done:
- Compiled with docs enabled
- Verified `build/doc/reference/pidgin/html/pidgin-cellrendererexpander.html` exists and has no mentions to gtkcellrendererexpander

Reviewed at https://reviews.imfreedom.org/r/203/