Clarify error messaging, group names must be provided. (#7529)

Signed-off-by: Callum Styan <callumstyan@gmail.com>

pkg/rulefmt/rulefmt.go

@@ -68,7 +68,7 @@ func (g *RuleGroups) Validate(node ruleGroups) (errs []error) {
 
 	for j, g := range g.Groups {
 		if g.Name == "" {
-			errs = append(errs, errors.Errorf("%d:%d: Groupname should not be empty", node.Groups[j].Line, node.Groups[j].Column))
+			errs = append(errs, errors.Errorf("%d:%d: Groupname must not be empty", node.Groups[j].Line, node.Groups[j].Column))
 		}
 
 		if _, ok := set[g.Name]; ok {