Update GETTING_STARTED.md with fenced code blocks

[steffen]

Please make sure to read the Contributing Guidelines:

https://github.com/neomjs/neo/blob/dev/CONTRIBUTING.md

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe: Markdown formatting

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch, not the master branch
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

I recommend using fenced code blocks (and code highlighting) for markdown documentation files, see https://help.github.com/en/github/writing-on-github/creating-and-highlighting-code-blocks.