docs: Improve markdown format in README.md

There should be a blank line after the headings

Author: PeterDaveHello

README.md

@@ -193,11 +193,13 @@ image as a base, add the things you need in your own Dockerfile
 examples of how to install packages if you are unfamiliar).
 
 ### `node:buster`
+
 This image is based on version 10 of
 [Debian](http://debian.org), available in
 [the `debian` official image](https://hub.docker.com/_/debian).
 
 ### `node:stretch`
+
 This image is based on version 9 of
 [Debian](http://debian.org), available in
 [the `debian` official image](https://hub.docker.com/_/debian).