Improve documentation

Author: oli-obk

src/ci/docker/README.md

@@ -20,7 +20,7 @@ Images will output artifacts in an `obj` dir at the root of a repository.
 
 - Each directory, excluding `scripts` and `disabled`, corresponds to a docker image
 - `scripts` contains files shared by docker images
-- `disabled` contains images that are not build travis
+- `disabled` contains images that are not built on travis
 
 ## Cross toolchains