Change documentation references from topic to topicArn

[asionek]

Description

Minor adjustment to the README to reflect the correct configuration of plugin.

This is needed because topic is not a valid property. It has to be topicArn.

 Configuration error at 'custom.slicWatch': unrecognized property 'topic'

[coveralls]

Coverage remained the same at 100.0% when pulling a6c8439 on asionek:main into efcea1a on fourTheorem:main.

[eoinsha]

Thanks for finding this and resolving it, @asionek 🙏