Fix typo creating QRbitstream()

Summary:
Make the one new QRbitrtream() look like the other nine new QRbitstream().

Interestingly, this bug was reported upstream since 2018, and still not fixed:

https://github.com/t0k4rt/phpqrcode/issues/47

Test Plan:
Read the code aloud and realize that this word sound weird.

Verify that the class QRbitrtream does not exist, and QRbitstream exists.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25651