Update pluggable transports link to tb-manual
In this support entry: "Several countries, including China and Iran, have found ways to detect and block connections to Tor bridges. Obfsproxy bridges address this by adding another layer of obfuscation. Setting up an obfsproxy bridge requires an additional software package and additional configurations. See our page on pluggable transports for more info."
We need to change the link to tb-manual pluggable transports section.