Update import error content

This content has been suggested by our content designer to clarify how
the row indexing works.

Author: thomasleese

app/views/immunisation_imports/errors.html.erb

@@ -5,12 +5,13 @@
       ) %>
 <% end %>
 
-<% title = "The vaccinations could not be added" %>
+<% title = "Vaccination records cannot be uploaded" %>
 
 <%= h1 title, page_title: "#{@campaign.name} – #{title}" %>
 
-<p>
-  It wasn’t possible to add the vaccinations due to the following errors in the uploaded CSV file:
+<p class="nhsuk-body">
+  The records cannot be uploaded due to errors in the CSV file.
+  When fixing these errors, note that the header does not count as a row.
 </p>
 
 <% @immunisation_import.errors.each do |error| %>

spec/features/immunisation_imports_upload_spec.rb

@@ -96,7 +96,7 @@ def when_i_upload_an_invalid_file
   end
 
   def then_i_should_see_the_errors_page
-    expect(page).to have_content("The vaccinations could not be added")
+    expect(page).to have_content("Vaccination records cannot be uploaded")
     expect(page).to have_content("Row 1")
   end