Add zk-spaced to list of related projects

Also take the opportunity to unify the dashes.

README.md

@@ -94,5 +94,6 @@ $ ./zk -h
 ## Related projects
 
 * [Neuron](https://github.com/srid/neuron) – a great tool to publish a Zettelkasten on the web
-* [Emanote](https://emanote.srid.ca/) - an improved successor to Neuron
+* [Emanote](https://emanote.srid.ca/) – an improved successor to Neuron
 * [sirupsen's zk](https://github.com/sirupsen/zk) – a collection of scripts with a similar purpose
+* [zk-spaced](https://github.com/matze/zk-spaced) – spaced repetition plugin for zk