Show address line 7 not postcode in example spreadsheet

Since the letter preview will now show address line 7, the example
spreadsheet we give should match.

app/main/views/send.py

@@ -20,7 +20,7 @@ from notifications_utils.columns import Columns
 from notifications_utils.pdf import is_letter_too_long
 from notifications_utils.postal_address import (
     PostalAddress,
-    address_lines_1_to_6_and_postcode_keys,
+    address_lines_1_to_7_keys,
 )
 from notifications_utils.recipients import RecipientCSV, first_column_headings
 from notifications_utils.sanitise_text import SanitiseASCII
@@ -63,7 +63,7 @@ from app.utils import (
 
 letter_address_columns = [
     column.replace('_', ' ')
-    for column in address_lines_1_to_6_and_postcode_keys
+    for column in address_lines_1_to_7_keys
 ]
 
 
@@ -102,7 +102,7 @@ def get_example_letter_address(key):
     return {
         'address line 1': 'A. Name',
         'address line 2': '123 Example Street',
-        'postcode': 'XM4 5HQ'
+        'address line 3': 'XM4 5HQ'
     }.get(key, '')
 
 
@@ -381,7 +381,7 @@ def send_one_off_letter_address(service_id, template_id):
         if all_placeholders_in_session(placeholders):
             return get_notification_check_endpoint(service_id, template)
 
-        first_non_address_placeholder_index = len(address_lines_1_to_6_and_postcode_keys)
+        first_non_address_placeholder_index = len(address_lines_1_to_7_keys)
 
         return redirect(url_for(
             'main.send_one_off_step',
@@ -475,7 +475,7 @@ def send_test_step(service_id, template_id, step_index):
 
     # if we're in a letter, we should show address block rather than "address line #" or "postcode"
     if template.template_type == 'letter':
-        if step_index < len(address_lines_1_to_6_and_postcode_keys):
+        if step_index < len(address_lines_1_to_7_keys):
             return redirect(url_for(
                 '.send_one_off_letter_address',
                 service_id=service_id,