Update access to encrypted volumes documentation for Jessie

When plugging a new USB stick, it does not appears in ‘Places’ unless you access it through ‘Computer’, so https://tails.boum.org/doc/encryption_and_privacy/encrypted_volumes/index.en.html#index4h1 needs to be updated.

Parent Task: #8043 (closed)

Original created by @intrigeri on 10810 (Redmine)