Add and apply RSpec/ContextMethod rubocop rule

This rule is available since rubocop-rspec 1.36.0. We were already
applying it in most cases.

spec/models/local_census_records/import_spec.rb

@@ -43,7 +43,7 @@ describe LocalCensusRecords::Import do
     end
   end
 
-  context "#save" do
+  describe "#save" do
     it "Create valid local census records with provided values" do
       import.save!
       local_census_record = LocalCensusRecord.find_by(document_number: "X11556678")