Group: Fix an error when update member's role

"A group have to have at least one admin" error occured when update
a group member's role to "member" if the user who updates was the last
group admin.

The error should occur if the updated user, not the user who updates,
was the last admin.

app/controllers/OrganizationApp.java

@@ -240,7 +240,8 @@ private static Result validateForEditMember(Form<Role> roleForm, String organiza
             return okWithLocation(routes.OrganizationApp.members(organizationName).url());
         }
 
-        if (organization.isLastAdmin(currentUser) && roleForm.get().id.equals(RoleType.ORG_MEMBER.roleType())) {
+        if (organization.isLastAdmin(User.find.byId(userId))
+                && roleForm.get().id.equals(RoleType.ORG_MEMBER.roleType())) {
             flash(Constants.WARNING, "organization.member.atLeastOneAdmin");
             return okWithLocation(routes.OrganizationApp.members(organizationName).url());
         }