Update content for date of vaccination inclusion error (#3106)

This is the improved content that has been suggested by our content
designers.

config/locales/en.yml

@@ -258,7 +258,7 @@ en:
               blank: Enter a date in the correct format
               greater_than: The vaccination date is before the date of birth
               less_than_or_equal_to: The vaccination date is in the future
-              inclusion: Enter a date for a current session
+              inclusion: Enter a date that matches when the vaccination session took place
             delivery_site:
               blank: Enter an anatomical site.
               inclusion: Enter a anatomical site that is appropriate for the vaccine.

spec/models/immunisation_import_row_spec.rb

@@ -211,7 +211,11 @@
 
         let(:session) { create(:session, organisation:, programmes:) }
 
-        it { should include(/current session/) }
+        it do
+          expect(errors).to include(
+            "Enter a date that matches when the vaccination session took place"
+          )
+        end
       end
 
       context "when importing without a session" do