docs(readme): fix syntax highlighting

[Splaktar]

• Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/preboot/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What modules are related to this pull-request

• server side
• client side
• inline
• build process
• docs
• tests

• What kind of change does this PR introduce?
  docs update

• What is the current behavior?
  Some of the code examples don't have syntax highlighting.
  Others use both ts and typescript.

• What is the new behavior (if this is a feature change)?
  All code examples have syntax highlighting.
  All example consistently use ts.

• Does this PR introduce a breaking change?
  No

• Other information: