expand.1: Fix markup for the '-t' option

When viewing the manpage the '-t' option is shown as follows

    -t -Sm tab1, tab2, ..., tabn Sm

with the markup '\&Sm' included within '.It Fl t', which doesn't
makes any sense.

So just remove it.

PR:	274897
MFC after:	3 days

usr.bin/expand/expand.1

@@ -78,7 +78,7 @@ If the
 .Fl a
 option is given, then tabs are inserted whenever they would compress the
 resultant file by replacing two or more characters.
-.It Fl t \&Sm Ar tab1 , tab2 , ... , tabn \&Sm
+.It Fl t Ar tab1 , tab2 , ... , tabn
 Set tab stops at column positions
 .Ar tab1 , tab2 , ... , tabn .
 If only a single number is given, tab stops are set that number of