chore(lint): Fix sentance/sentence codespell warning (#4165)

locales/en-US/strings.properties

@@ -184,7 +184,7 @@ firstrun_content=Get your bookmarks, history, passwords and other settings on al
 firstrun_learn_more_link=Learn more about Firefox Accounts
 
 # LOCALIZATION NOTE (firstrun_form_header and firstrun_form_sub_header):
-# firstrun_form_sub_header is a continuation of firstrun_form_header, they are one sentance.
+# firstrun_form_sub_header is a continuation of firstrun_form_header, they are one sentence.
 # firstrun_form_header is displayed more boldly as the call to action.
 firstrun_form_header=Enter your email
 firstrun_form_sub_header=to continue to Firefox Sync.