fix typo in "readonly" flag in documentation

the flag is named '--read-only', not '--readonly'

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit ac12696ff48aeb115e3d9ce3b13cfa54342b5aee)
Signed-off-by: Tibor Vass <tibor@docker.com>

docs/reference/commandline/dockerd.md

@@ -911,7 +911,7 @@ The following standard Docker features are currently incompatible when
 running a Docker daemon with user namespaces enabled:
 
  - sharing PID or NET namespaces with the host (`--pid=host` or `--net=host`)
- - A `--readonly` container filesystem (this is a Linux kernel restriction against remounting with modified flags of a currently mounted filesystem when inside a user namespace)
+ - A `--read-only` container filesystem (this is a Linux kernel restriction against remounting with modified flags of a currently mounted filesystem when inside a user namespace)
  - external (volume or graph) drivers which are unaware/incapable of using daemon user mappings
  - Using `--privileged` mode flag on `docker run` (unless also specifying `--userns=host`)