Fix a typo in the install docs. (#9040)

INSTALL.md

@@ -257,7 +257,7 @@ for a number of platforms.
 
 #### Docker images and Ansible playbooks
 
-There is an offical synapse image available at
+There is an official synapse image available at
 <https://hub.docker.com/r/matrixdotorg/synapse> which can be used with
 the docker-compose file available at [contrib/docker](contrib/docker). Further
 information on this including configuration options is available in the README

changelog.d/9040.doc

+Corrected a typo in `INSTALL.md`.