doc: git-reset: make "<mode>" optional

The git-reset's "<mode>" is an optional argument, however it was
documented as required.

The "<mode>" is documented as one of: --soft, --mixed, --hard, --merge
or --keep, so "<mode>" should be used instead of "--<mode>".

Signed-off-by: Krzysztof Mazur <krzysiek@podlesie.net>
Signed-off-by: Jeff King <peff@peff.net>

Documentation/git-reset.txt

@@ -10,7 +10,7 @@ SYNOPSIS
 [verse]
 'git reset' [-q] [<commit>] [--] <paths>...
 'git reset' (--patch | -p) [<commit>] [--] [<paths>...]
-'git reset' (--soft | --mixed | --hard | --merge | --keep) [-q] [<commit>]
+'git reset' [--soft | --mixed | --hard | --merge | --keep] [-q] [<commit>]
 
 DESCRIPTION
 -----------
@@ -43,11 +43,11 @@ This means that `git reset -p` is the opposite of `git add -p`, i.e.
 you can use it to selectively reset hunks. See the ``Interactive Mode''
 section of linkgit:git-add[1] to learn how to operate the `--patch` mode.
 
-'git reset' --<mode> [<commit>]::
+'git reset' [<mode>] [<commit>]::
 	This form resets the current branch head to <commit> and
 	possibly updates the index (resetting it to the tree of <commit>) and
-	the working tree depending on <mode>, which
-	must be one of the following:
+	the working tree depending on <mode>. If <mode> is omitted,
+	defaults to "--mixed". The <mode> must be one of the following:
 +
 --
 --soft::