Improve custom ranking rules example text

[guimachiavelli]

The last line of the example text under "Ranking rules > Custom rules" in /learn/core_concepts/relevancy.mdx reads:

To add a rule to the existing ranking rule, you have to add the rule to the existing ordered rules array using the settings route,

This is very difficult to parse due to the repetition of the same words referring to different things in very quick succession. Though this might be unavoidable to some extent, we should be able to rephrase it so it's easier to read.