stf.4: Remove spurious word

Improve grammar by removing spurious word.

MFC after:	3 days
Approved by:	emaste, carlavilla (mentor)
Pull Request:	#1605

Author: msimerson

share/man/man4/stf.4

@@ -201,7 +201,7 @@ The IPv4 WAN address.
 The prefix length of the IPv4 WAN address.
 .El
 .Pp
-These can parameters are all configured through
+These parameters are all configured through
 .Xr ifconfig 8 .
 .Pp
 The IPv6 address and prefix length can be configured like any other IPv6 address.