update hroller man page: node group filtering is implemented

Filtering of nodes by node group was already implemented in
2fce67b6; fix the man page to reflect this change.

Signed-off-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>

man/hroller.rst

@@ -21,6 +21,8 @@ Backend options:
 
 Algorithm options:
 
+**[ -G *name* ]**
+
 **[ -O *name...* ]**
 
 Reporting options:
@@ -59,8 +61,6 @@ somehow easily identifiable. Right now this is not done.
 
 Offline nodes should be ignored.
 
-Filtering by nodegroup should be allowed.
-
 If instances are online the tool should refuse to do offline rolling
 maintenances, unless explicitly requested.