docs(core): fix toSignal docs

[garrettld]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/main/CONTRIBUTING.md#commit

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

toSignal and ToSignalOptions API docs mistakenly say "toObservable" instead of "toSignal"

What is the new behavior?

toSignal and ToSignalOptions API docs say "toSignal"

Does this PR introduce a breaking change?

• No

[atscott]

This PR was merged into the repository by commit b87b2fc.

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