Corrected highlight for zpool man page

SS is already highlighted and the fB/fR tags break the highlighting
prematurely, removing the tags highlights the entire line.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #5873

man/man8/zpool.8

@@ -184,7 +184,7 @@ The \fBzpool\fR command configures \fBZFS\fR storage pools. A storage pool is a
 .sp
 .LP
 All datasets within a storage pool share the same space. See \fBzfs\fR(8) for information on managing datasets.
-.SS "Virtual Devices (\fBvdev\fRs)"
+.SS "Virtual Devices (vdevs)"
 .sp
 .LP
 A "virtual device" describes a single device or a collection of devices organized according to certain performance and fault characteristics. The following virtual devices are supported: