docs(model): correct TreeModel state subscription documentation

[KeithGillette]

Documentation incorrectly uses generic subscribe instead of appropriate subscribeToState method

PR Checklist

Please check if your PR fulfills the following requirements:

• [X] The commit message follows our guidelines: https://github.com/CirclonGroup/angular-tree-component/blob/master/CONTRIBUTING.md#commit-message-guidelines
• [N/A] Tests for the changes have been added (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[X] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Documentation references and shows example of subscribe method but uses signature of subscribeToState method.

What is the new behavior?

Corrects references & example of generic subscribe to subscribeToState method.

Does this PR introduce a breaking change?

[ ] Yes
[X] No

Other information