docs(pdk) fix name of request.get_query_arg function
docs(pdk) fix name of request.get_query_arg function

Manually fix the typo (fixed in Kong/kong@115970) because we cannot
rebuild the PDK docs from the master branch based on the current
implementation of `gulp pdk-docs`.

Mashape/getkong.org,Mashape/getkong.org,Mashape/getkong.org