docs: fix table in comparing observables guide

[ro-savage]

Fix table in comparing-observables.md that was incorrectly formatted.

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
[x] Documentation content changes
[ ] angular.io application / infrastructure changes
[ ] Other... Please describe:

What is the current behavior?

Table columns are off by one.

Issue Number: N/A

What is the new behavior?

Table formatted correctly.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

[ngbot]

Hi @ro-savage! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[mary-poppins]

You can preview 9e492ec at https://pr22485-9e492ec.ngbuilds.io/.

[brandonroberts]

Update the git commit message to docs: fix table formatting in comparing observables guide

[mary-poppins]

You can preview 95151a1 at https://pr22485-95151a1.ngbuilds.io/.

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