Update docs/hooks/hook_civicrm_alterMailParams.md

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

docs/hooks/hook_civicrm_alterMailParams.md

@@ -41,7 +41,7 @@ You can add custom headers by appending to `$params['headers']`. Example:
     $params['headers']['X-My-Header'] = 'my header value';
     ```
 
-Note: the `headers` key may not exist in the `$params` array when passed into
+The `headers` key may not exist in the `$params` array when passed into the hook.
 
 For CiviMail based emails you can also add headers by simply adding a key
 directly to the `$params` array, however as CiviMail also supports the above, so