Fix Usage as a remark plugin without Gatsby example code#229

The example in the Usage as a remark plugin without Gatsby section does not compile as is: - Top-level await only available in ES Modules but used here with require() – decided to go with the former and use imports - Backticks not escaped in template string - Incorrect module name (`remarkParse` should be `remark-parse`) - Incorrect property reference in result returned from processor (`contents` should be `value`) I also added the npm install line required and a suggestion of the file name to use (including _.mjs_ extension) to make it easier to follow and avoid possible trip-ups.