Make email address for bridges consistent

BridgeDB claims that users should email bridges[at]torproject.org but our website mentions bridges[at]bridges.torproject.org. This confused at least one user over at legacy/trac#18076 (moved) and, frankly, me too.

I created a patch in the following GitHub branch: https://github.com/NullHypothesis/webml/tree/update-bridge-docs

It makes our documentation consistent by changing the email address to bridges[at]torproject.org.