xproto: Fix GContext error references in documentation
xproto: Fix GContext error references in documentation

The name of the error is GContext, not GC.

Signed-off-by: Daniel Martin <5ae2633454ceb931c1fd86ffad586b4dfc721833@gmail.com>
Reviewed-By: Ran Benita <03686f031484546e32da38cdd36b1b8bb5e2be3c@gmail.com>

nbaum/xcb-proto,nbaum/xcb-proto