docs(messageformat): update installation command

[mibnd]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: https://github.com/ngneat/transloco/blob/master/CONTRIBUTING.md#commit
• [ ] Tests for the changes have been added (for bug fixes / features)
• [x] 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
[ ] Other... Please describe:

What is the current behavior?

Install command adds old messageformat package which is no more needed. New @messageformat/core package has been added to dependencies earlier.

npm i messageformat @ngneat/transloco-messageformat

Issue Number: N/A

What is the new behavior?

Installation @ngneat/transloco-messageformat without adding messageformat to package.json.

npm i @ngneat/transloco-messageformat

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information