Add and apply Naming/VariableName rubocop rule

We forgot to use it in one place, and we've found out other institutions
using CONSUL whose developers aren't so familiar with Ruby also break
this rule, so it might be better to add it explicitly.

spec/system/sdg_management/targets_spec.rb

@@ -21,7 +21,7 @@ describe "Targets" do
       goal_8_target_10 = SDG::Target["8.10"]
       goal_16 = SDG::Goal[16]
       goal_16_target_10 = SDG::Target["16.10"]
-      goal_16_target_A = SDG::Target["16.A"]
+      goal_16_target_a = SDG::Target["16.A"]
 
       visit sdg_management_targets_path
 
@@ -29,7 +29,7 @@ describe "Targets" do
       expect(goal_8_target_2.title).to appear_before(goal_8_target_10.title)
       expect(goal_8_target_10.title).to appear_before(goal_16.title)
       expect(goal_16.title).to appear_before(goal_16_target_10.title)
-      expect(goal_16_target_10.title).to appear_before(goal_16_target_A.title)
+      expect(goal_16_target_10.title).to appear_before(goal_16_target_a.title)
     end
   end
 end