Update instructions to reflect the use of new signing subkey

In the "Verifying the signature" section on https://support.torproject.org/tbb/how-to-verify-signature/, subkey EB774491D9FF06E2 is still shown in the example output to validate the signature, whereas E53D989A9E2D47BF is the actual subkey used to do the actual signing by now.

I do understand that the commands are not to be taken literal, but aim to describe the general approach, however, gpg signing is a sensitive topic, and confusing enough as it is for new users, updating the instructions might help to prevent some unnecessary doubt.