Minor with-docker readme grammar fix (#11760)

* Fix with-docker readme grammar

* Amended with-docker readme grammar fix
Co-Authored-By: NLuis Alvarez D. <luis@zeit.co>
Co-authored-by: NLuis Alvarez D. <luis@zeit.co>

examples/with-docker/README.md

 # With Docker
 
-This example show how to set custom environment variables for your **docker application** at runtime.
+This example shows how to set custom environment variables for your **docker application** at runtime.
 
 The `dockerfile` is the simplest way to run Next.js app in docker, and the size of output image is `173MB`. However, for an even smaller build, you can do multi-stage builds with `dockerfile.multistage`. The size of output image is `85MB`.