doc-rst: linux_tv: Fix some occurences of :sub:

The right way to use it seems to do suscript is to use
this pattern: "\ :sub:"

Make sure all places of the media document will fit, by
using this script:

$n=0;
while (<>) {
	$n++;
	$t = $_;
	@matches = $t =~ m/(..\:sub\:)/g;
	foreach my $m (@matches) {
		$m =~ m/(.)(.)(\:sub\:)/;
		$s1=$1;
		$s2=$2;
		$s3=$3;
		next if (($s1 eq "\\") && ($s2 eq " "));
		if ($s2 eq " ") {
			$t =~ s/$s1$s2$s3/$s1\\$s2$s3/;
			next;
		}
		$t =~ s/$s1$s2$s3/$s1$s2\\ $s3/;
	}
	print $t;
}

And running it with:

for i in $(git grep -l sub Documentation/linux_tv/); do ./sub.pl $i >a && mv a $i; done

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>