taskset: fix description of `-c` option in the man page

taskset: fix description of `-c` option in the man page

Commit 42f15e9848dc07e86a2b80c742a0179aaced6c71 introduced a certain
inaccuracy in -c option description; it is not _the option with an argument_,
but it is the flag that changes interpretation of `mask` parameter.
The difference is almost not noticeable, but there are one corner case where it is:
   taskset -c 1 -p $$
fails (but it would work if 42f15e was correct). Instead,
   taskset -c -p 1 $$
works (but it should not have if 42f15e was correct).

Signed-off-by: Karel Zak <kzak@redhat.com>