doc: Add missing symbol in cogl_framebuffer_get_context() documentation
doc: Add missing symbol in cogl_framebuffer_get_context() documentation

(cherry picked from commit 146a1bfc2655c7e7ed0a5a4fd64b490385159f49)

Distrotech/cogl,Distrotech/cogl,Distrotech/cogl,Distrotech/cogl