geom: minor man page updates suggested by igor(1)

Reviewed by:	pauamma
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D37681

sbin/geom/core/geom.8

@@ -62,10 +62,10 @@ The
 .Nm
 utility is used to control various GEOM classes.
 A class has to be aware of
-.Xr geom 8
+.Nm
 communication methods, but there are also some standard commands
 which can be used for existing
-.Xr geom 8
+.Nm
 unaware classes.
 Here is the list of standard commands:
 .Bl -tag -width ".Cm status"
@@ -128,7 +128,7 @@ available under the name of
 .Nm g Ns Ar class .
 .Pp
 Currently available classes which are aware of
-.Xr geom 8 :
+.Nm :
 .Pp
 .Bl -bullet -offset indent -compact
 .It