Replace userid with fingerprint in HACKING.md
Replace userid with fingerprint in HACKING.md

We remove spaces in the key identifier in crypto.py,
so using the gpg userid as the key identifier instead of
the fingerprint won't work. See
https://github.com/freedomofpress/securedrop/issues/96.

pwplus/securedrop,jeann2013/securedrop,GabeIsman/securedrop,GabeIsman/securedrop,pwplus/securedrop,conorsch/securedrop,GabeIsman/securedrop,jrosco/securedrop,jrosco/securedrop,GabeIsman/securedrop,pwplus/securedrop,jaseg/securedrop,kelcecil/securedrop,pwplus/securedrop,jeann2013/securedrop,garrettr/securedrop,jaseg/securedrop,GabeIsman/securedrop,ehartsuyker/securedrop,jrosco/securedrop,ageis/securedrop,jrosco/securedrop,kelcecil/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,jaseg/securedrop,harlo/securedrop,jaseg/securedrop,harlo/securedrop,kelcecil/securedrop,jrosco/securedrop,jeann2013/securedrop,garrettr/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,jeann2013/securedrop,chadmiller/securedrop,harlo/securedrop,kelcecil/securedrop,harlo/securedrop,ehartsuyker/securedrop,harlo/securedrop,jaseg/securedrop,jeann2013/securedrop,micahflee/securedrop,pwplus/securedrop,conorsch/securedrop,conorsch/securedrop,ageis/securedrop,chadmiller/securedrop,ageis/securedrop,chadmiller/securedrop,harlo/securedrop,jaseg/securedrop,chadmiller/securedrop,garrettr/securedrop,jrosco/securedrop,ehartsuyker/securedrop,jeann2013/securedrop,heartsucker/securedrop,kelcecil/securedrop,heartsucker/securedrop,conorsch/securedrop,micahflee/securedrop,garrettr/securedrop,chadmiller/securedrop,micahflee/securedrop,heartsucker/securedrop,GabeIsman/securedrop,kelcecil/securedrop,ehartsuyker/securedrop,chadmiller/securedrop,pwplus/securedrop,micahflee/securedrop,ageis/securedrop,conorsch/securedrop