[doc] Fix two small typos in cogl-vertex-buffer
[doc] Fix two small typos in cogl-vertex-buffer

The symbol name for cogl_vertex_buffer_draw_elements was wrong so it
ended up with no documentation. The name for the vertex attribute was
incorrect in cogl_vertex_buffer_add.

Distrotech/clutter,Distrotech/clutter,GNOME/clutter,collects/clutter,GNOME/clutter,kerrickstaley/clutter-vala,rkudiyarov/clutter_osx,collects/clutter,jigpu/clutter,djdeath/clutter,rkudiyarov/clutter_osx,djdeath/clutter-android,dlespiau/clutter,jigpu/clutter,kerrickstaley/clutter-vala,ebassi/clutter,heysion/clutter-clone,djdeath/clutter-android,djdeath/clutter-multithreaded,jigpu/clutter,ebassi/clutter,nobled/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,collects/clutter,djdeath/clutter,tardyp/clutter,spatulasnout/clutter,heysion/clutter-clone,jigpu/clutter,collects/clutter,djdeath/clutter-multithreaded,djdeath/clutter-android,ebassi/clutter,djdeath/clutter,djdeath/clutter,djdeath/clutter-android,djdeath/clutter,tardyp/clutter,djdeath/clutter-android,spatulasnout/clutter,djdeath/clutter-multithreaded,Distrotech/clutter,tardyp/clutter,GNOME/clutter,dlespiau/clutter,Distrotech/clutter,Distrotech/clutter,nobled/clutter,spatulasnout/clutter,dlespiau/clutter,spatulasnout/clutter,GNOME/clutter,jigpu/clutter,rkudiyarov/clutter_osx,djdeath/clutter,ebassi/clutter,kerrickstaley/clutter-vala,ebassi/clutter,spatulasnout/clutter,nobled/clutter,heysion/clutter-clone,kerrickstaley/clutter-vala,heysion/clutter-clone,jigpu/clutter,heysion/clutter-clone,GNOME/clutter,collects/clutter,rkudiyarov/clutter_osx