Fix a small typo (#160)

docs/upgrading_3_4.rst

@@ -4,7 +4,7 @@ Upgrading from python-fints 3.x to 4.x
 Release 4.0 of this library was made to introduce a breaking change:
 
 * You now need to register your application with the Deutsche Kreditwirtschaft (German banking association) and supply
-  your assigned product IT when initializing the library.
+  your assigned product ID when initializing the library.
 
 The library used to have a built-in product ID that was used as a default if you didn't. This was very useful, but
 Deutsche Kreditwirtschaft asked us to stop doing this, since it undermindes the whole point of the product registration.