Update slices-pattern.md - add link to Persist middleware page (#3335)

* Update slices-pattern.md - add link to Persist middleware page

I thought it would be nice to have a link next to the topic

* Update slices-pattern.md - add link to Persist middleware page

Change to relative path

* Update slices-pattern.md - correct link to Persist middleware page

correct link to Persist middleware page to relative path