Fix documentation syntax of optional arguments in short options.

Fix documentation syntax of optional arguments in short options.

When an argument for an option is optional, like in -n from git-tag,
puting a space between the option and the argument is interpreted
as a missing argument for the option plus an isolated argument.
Documentation now reflects the need to write the parameter following
the option -n, as in "git tag -nARG", for instance.

Signed-off-by: Carlos Rica <jasampler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>