README.adoc: update/correct the build procedure (#178)

Author: stephengold

README.adoc

@@ -17,14 +17,12 @@ NOTE: Read the link:https://wiki.jmonkeyengine.org/docs-wiki/3.4/wiki_contributo
 
 == Build & Preview
 
-To set up the Antora environment, you'll need Nodejs (tested with node 12).
+To set up the Antora environment, you'll need Node.js and Nvm.
 
-From your local wiki directory.
-
-Run:
+From your local wiki directory, run:
 ```
 npm install
-npm run buildDocs
+npx antora wiki-playbook.yml
 ```
 
 This will install the needed dependencies and run the static site generator. The documentation will be output to the directory `build/site`.