Spurious warning

This produces a warning when you write `[br]` in a QBK file:

https://github.com/boostorg/quickbook/blob/5f2e8c24cb813b4b0b94d3316e664b971c5b71dd/src/actions.cpp#L265

Thing is... it works fine. There is no other "boostbook processor" on the horizon, so the warning is rather pointless. We want to make doc warnings into errors in our CI builds but this is in the way.