commit dff79e27 renamed
the (small object) "git_sobj" to a plain "git_obj", but
neglected to update some of the documentation to reflect
that change.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>