docs: correct the names of the authentication headers

The authentication header names were incorrect.

The header names are defined in:
lib/WebService/WebServiceHeaders.php

doc/API.md

@@ -26,9 +26,9 @@ For all of the secure service calls it is required to be
 1. The result from the [Authenticate](#Authenticate) POST API call, if
    successful, will contain the two values: `sessionToken` and `userId`
 1. When making secure service calls the following headers must be set:
-    1.  `X-LibreBooking-SessionToken` set to the value of `sessionToken`
+    1.  `X-Booked-SessionToken` set to the value of `sessionToken`
         returned by the [Authenticate](#Authenticate) API call.
-    1.  `X-LibreBooking-UserId` set to the value of `userId` returned by the
+    1.  `X-Booked-UserId` set to the value of `userId` returned by the
         [Authenticate](#Authenticate) API call.
 
 ### POST Requests