Update the label of sebastianrakel's key

It looks like Sebastian Rakel current GitHub handle is "sebastianrakel",
and I was quite confused while reading this file about this
"Spritzgebaeck".  The key fingerprint correspond to Sebastian's one
(available at https://github.com/sebastianrakel.gpg), so I guess his
username has been misfiled, or Sebastian has changed it after the file
was created.

Since other keys seems to be "labelled" with the owner's current
username on GitHub, adjust the key "label" accordingly.

These labels seems to be informational only so no side effect is expected.

recipients.txt

@@ -1,5 +1,5 @@
 0xC10B6298A584A5632E254DA304D659E6BF1C4CC0 bastelfreak
 0x559BEE876892AA5ECD925F28B352E2A17BB89EF5 ewoud
-0x2b04d7500fe1c6dd15692bb6779eba5eb9d631b8 Spritzgebaeck
+0x2b04d7500fe1c6dd15692bb6779eba5eb9d631b8 sebastianrakel
 0x82349A78E7C0B8070B5980FFBA4D1D955112336F smortex
 0x83CE67EC15FA9327838924766CE2B38A165F224B rwaffen