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>

parse-options.c

@@ -330,7 +330,7 @@ void usage_with_options_internal(const char * const *usagestr,
 		switch (opts->type) {
 		case OPTION_INTEGER:
 			if (opts->flags & PARSE_OPT_OPTARG)
-				pos += fprintf(stderr, " [<n>]");
+				pos += fprintf(stderr, "[<n>]");
 			else
 				pos += fprintf(stderr, " <n>");
 			break;