merge-strategies: avoid implying that "-s theirs" exists

merge-strategies: avoid implying that "-s theirs" exists

The description of `-Xours` merge option has a parenthetical note
that tells the readers that it is very different from `-s ours`,
which is correct, but the description of `-Xtheirs` that follows it
carelessly says "this is the opposite of `ours`", giving a false
impression that the readers also need to be warned that it is very
different from `-s theirs`, which in reality does not even exist.

Clarify it a bit to avoid misleading readers.

Signed-off-by: Junio C Hamano <gitster@pobox.com>