docs: Updated inject example to show constructor usage

[ajitzero]

PR Checklist

Please check if your PR fulfills the following requirements:

• [x] The commit message follows our guidelines: CONTRIBUTING.md#commit
• [x] 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?

Duplicate example in https://ngworker.github.io/lumberjack/docs/usage#using-the-lumberjackservice where both show usage with inject().

Issue Number: N/A

What is the new behavior?

Updated the first example to use the constructor for injection.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

Fixes #163

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[NachoVazquez]

Thanks for the PR @ajitzero ! Very appreciated.

I'll take a look right now.

[NachoVazquez]

@all-contributors please add @ajitzero for docs

[allcontributors[bot]]

@NachoVazquez

I've put up a pull request to add @ajitzero! :tada: