Correct Synapse install command for FreeBSD. (#11267)

Co-authored-by: reivilibre <olivier@librepush.net>

changelog.d/11267.doc

+Update Synapse install command for FreeBSD as the package is now prefixed with `py38`. Contributed by @itchychips.

docs/setup/installation.md

@@ -164,7 +164,7 @@ xbps-install -S synapse
 Synapse can be installed via FreeBSD Ports or Packages contributed by Brendan Molloy from:
 
 - Ports: `cd /usr/ports/net-im/py-matrix-synapse && make install clean`
-- Packages: `pkg install py37-matrix-synapse`
+- Packages: `pkg install py38-matrix-synapse`
 
 #### OpenBSD