Fix README installation instructions (Fixes #20)

[JonZeolla]

Summary of the contribution

Fixes typos in the installation instructions.

Testing

N/A

Checklist

• [X] Confirm that any associated issues are indicated in the pull request title
• [X] Rebase your PR against the latest commit within the target branch
• [X] Include steps to verify and test the intended change
• [X] Write or update unit tests and/or integration tests to verify your changes
• [X] Run shellcheck against any new or updated shell scripts, indicating the reasoning behind any disabled checks
• [X] Indicate whether or not this is a breaking change
• [X] Ensure that any new dependencies are these dependencies licensed in a way that is compatible for inclusion under ASF 2.0

[ckreibich]

Nice README! I think a quick comment to state that ldconfig might be needed after manual librdkafka installation wouldn't hurt. (I checked the source tarball and while they have ldconfig invocations in there for RPMs and debs, I don't see one for manual installation.)

[davehouser1]

Agree with @ckreibich. Also I would recommended updating the README example sections. @load packages/zeek-kafka/Seiso/Kafka is listed throughout, but does not exist on install. Change to @load packages/zeek-kafka This is tested and works.