git-diff.txt: don't mark required argument as optional

git-diff.txt: don't mark required argument as optional

Commit b7e10b2ca2 ("Documentation: usage for diff combined commits",
2020-06-12) modified the synopsis by adding an optional "[<commit>...]"
to

  'git diff' [<options>] <commit> <commit> [--] [<path>...]

to effectively add

  'git diff' [<options>] <commit> <commit>... <commit> [--] [<path>...]

as another valid invocation. Which makes sense.

Further down, in the description, it left the existing entry for

  'git diff' [<options>] <commit> <commit> [--] [<path>...]

intact and added a new entry on

  'git diff' [<options>] <commit> [<commit>...] <commit> [--] [<path>...]

where it says that "[t]his form is to view the results of a merge
commit" and details how "the first listed commit must be the merge
itself". But one possible instantiation of this form is `git diff
<commit> <commit>` for which the added text doesn't really apply.

Remove the brackets so that we lose this overlap between the two
descriptions. We can still use the more compact representation in the
synopsis.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>