Add 'device cxgbe' explicitly in the synopsis.

ccr depends on symbols exported by the cxgbe driver as well as having
a runtime dependency.  While the runtime depenency was noted in the
manpage already, the compile-time dependency wasn't as clear.

PR:		238265
MFC after:	3 days
Sponsored by:	Chelsio Communications

share/man/man4/ccr.4

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd April 29, 2019
+.Dd June 3, 2019
 .Dt CCR 4
 .Os
 .Sh NAME
@@ -36,6 +36,7 @@ place the following lines in your
 kernel configuration file:
 .Bd -ragged -offset indent
 .Cd "device ccr"
+.Cd "device cxgbe"
 .Ed
 .Pp
 To load the driver as a