validate/CheckRoot: fix error text

"This field" is not very descriptive. Replace with field name.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

validate/validate.go

@@ -179,12 +179,12 @@ func (v *Validator) CheckRoot() (errs error) {
 			return
 		} else if v.spec.Root == nil {
 			errs = multierror.Append(errs,
-				specerror.NewError(specerror.RootOnWindowsRequired, fmt.Errorf("on Windows, for Windows Server Containers, this field is REQUIRED"), rspec.Version))
+				specerror.NewError(specerror.RootOnWindowsRequired, fmt.Errorf("on Windows, for Windows Server Containers, Root is REQUIRED"), rspec.Version))
 			return
 		}
 	} else if v.platform != "windows" && v.spec.Root == nil {
 		errs = multierror.Append(errs,
-			specerror.NewError(specerror.RootOnNonWindowsRequired, fmt.Errorf("on all other platforms, this field is REQUIRED"), rspec.Version))
+			specerror.NewError(specerror.RootOnNonWindowsRequired, fmt.Errorf("on all other platforms, Root is REQUIRED"), rspec.Version))
 		return
 	}