docs: add missing 'on' in Oracle installation guide

Signed-off-by: Hans Kristian Flaatten <hans.flaatten@evry.com>

docs/installation/linux/oracle.md

@@ -11,7 +11,7 @@ parent = "engine_linux"
 
 # Oracle Linux
 
-Docker is supported Oracle Linux 6 and 7. You do not require an Oracle Linux
+Docker is supported on Oracle Linux 6 and 7. You do not require an Oracle Linux
 Support subscription to install Docker on Oracle Linux.
 
 ## Prerequisites