Fix typo in tag_feed_spec.rb (#16466)

existant -> existent

spec/models/tag_feed_spec.rb

@@ -37,7 +37,7 @@ describe TagFeed, type: :service do
       expect(results).to     include both
     end
 
-    it 'handles being passed non existant tag names' do
+    it 'handles being passed non existent tag names' do
       results = described_class.new(tag1, nil, any: ['wark']).get(20)
       expect(results).to     include status1
       expect(results).to_not include status2