Update overlayfs-driver.md
Update overlayfs-driver.md

Change "referred to a" to "referred to as a"
thaJeztah/docker.github.io,docker/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,thaJeztah/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,londoncalling/docker.github.io,docker/docker.github.io,docker/docker.github.io,thaJeztah/docker.github.io,docker/docker.github.io