doc: Fix link to wl_list_remove()

Use function linking syntax instead of variable linking, to resolve two
warnings:

    wayland-server.h:167: warning: explicit link request to 'wl_list_remove' could not be resolved
    wayland-server.h:188: warning: explicit link request to 'wl_list_remove' could not be resolved

Signed-off-by: Bryce Harrington <b.harrington@samsung.com>