system: make systemd's kernel headers comment fit

The comment line that explains that systemd needs a glibc toolchain and
kernel headers >= 3.10 doesn't fit in the dialog box, which makes it
look as if headers >= 3.1 are needed.

Abbreviate the comment a little by removing an 'an'. It still doesn't
fit completely (only one trailing * is shown), but at least it's
readable.

While we're at it, change the quoting style to " like we always use.

Reported-by: eschu on IRC
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>