docs: Fix typo in singleton-services.md page

[gaborszekely]

Fix typo by replacing the word "once" with "one" on the Singleton Services documentation page.

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/angular/angular/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (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
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

There is a typo on the Singleton Services documentation page, on the very first line. The sentence reads "A singleton service is a service for which only once instance exists in an app", whereas it should read "A singleton service is a service for which only one instance exists in an app".

Issue Number: N/A

What is the new behavior?

Replace first sentence with the proper word.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[kapunahelewong]

Thanks for this, @gaborszekely!

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