Fix another typo in SHA attribute spec

spec/models/concerns/sha_attribute_spec.rb

@@ -13,7 +13,7 @@ describe ShaAttribute do
   end
 
   describe '#sha_attribute' do
-    context' when the table exists' do
+    context 'when the table exists' do
       before do
         allow(model).to receive(:table_exists?).and_return(true)
       end