Documentation: reword rebase summary

The wording is introduced in c3f0baac (Documentation: sync git.txt
command list and manual page title, 2007-01-18), but rebase has evolved
since then, capture the modern usage by being more generic about the
rebase command in the summary.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>