Fix documentation, instantiation example

[jr-k]

This is:

• a bugfix
• a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Minor documentation fix

[MarkBaker]

Hmm! Yes, thanks for correcting this.

I'm glad that some people actually read the documentation, especially when they notice silly errors like this