Add missing link to Handlebars.java

[eneveu]

It was present in the graph but not listed with the other template engines. This will improve SEO.

[eneveu]

Maybe we could also specify "Handlebars.java" and "Mustache.java" somewhere in the README, to improve SEO, and to make it clear that we are benchmarking those specific implementations?

Either:

• Handlebars (Handlebars.java implementation)
• Mustache (Mustache.java implementation)

Or:

• Handlebars.java
• Mustache.java