freebsd-base.7: Fix stray -r

Remote stray "-r" from the example of installing a toolchain
to alternate root.

Reviewed by:	ivy
Differential Revision:	https://reviews.freebsd.org/D54611

share/man/man7/freebsd-base.7

@@ -214,7 +214,7 @@ alternate root (for example, to support cross-compiling software
 for a different target than the host system):
 .Bd -literal -offset indent
 pkg -r /ppcdev -oABI=FreeBSD:16:powerpc64le \e
-    install -r FreeBSD-set-devel
+    install FreeBSD-set-devel
 .Ed
 .Sh SEE ALSO
 .Xr build 7 ,