Updates signing key in 0.3.5 -> 0.3.6 upgrade docs
Updates signing key in 0.3.5 -> 0.3.6 upgrade docs

These instructions are deprecated, and shouldn't be used by any
currently running instance, but we still need to update the signing key
fingerprint key to keep the documentation accurate.

micahflee/securedrop,ehartsuyker/securedrop,garrettr/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,micahflee/securedrop,ageis/securedrop,ehartsuyker/securedrop,garrettr/securedrop,micahflee/securedrop,ageis/securedrop,garrettr/securedrop,conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,micahflee/securedrop,ageis/securedrop,garrettr/securedrop,ageis/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop