Rename references to release signing key
Rename references to release signing key

0.3.10 rotates from using the "Freedom of the Press Foundation Master
Signing Key" to sign releases to a new project-specific "SecureDrop
Release Signing Key". This commit updates the name of the key (the
fingerprint had already been updated) in the install documentation to
avoid confusion.

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