matrix-synapse: New apache configuration has syntax errors

!1540 (closed) introduced a syntax validation problem to Apache configuration. Apache fails to start after the changes.

AH00526: Syntax error on line 9 of /etc/apache2/conf-enabled/matrix-synapse-plinth.conf:
AllowEncodedSlashes not allowed in <Location> context

Although I tested matrix after the configuration changes, I forgot to restart apache before testing.