fix company-type stop screen content (grammar)

match change to prototype V9

locales/en/stop-screens.json

@@ -2,11 +2,11 @@
   "company_type_main_title": "You cannot provide verification details for this company through this service",
   "company_type_main_heading": "Only certain company types can use this service",
   "company_type_intro_start": "You can only provide verification details for PSCs of",
-  "company_type_intro_end": "if it is a:",
-  "company_type_allowed_1": "private limited company",
-  "company_type_allowed_2": "public limited company",
-  "company_type_allowed_3": "unlimited company",
-  "company_type_allowed_4": "Community Interest Company (CIC)",
+  "company_type_intro_end": "if it is:",
+  "company_type_allowed_1": "a private limited company",
+  "company_type_allowed_2": "a public limited company",
+  "company_type_allowed_3": "an unlimited company",
+  "company_type_allowed_4": "a Community Interest Company (CIC)",
   "company_type_wrong_company_action": "If this is the wrong company, ",
   "company_type_wrong_company_action_text": "go back and enter a different company number",
   "company_type_contact_us_text": "Contact us",