'http.get' has been swapped in for 'of'

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?

Refers to swapping in 'of' for 'http.get' which is incorrect when referring to the code
Issue Number: N/A

What is the new behavior?

'http.get' in place of 'of' to reflect the changes in code

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information