Reflect 'project_services' -> 'integrations' namespace change

What does this MR do?

It's a trivial fix to a link where the directory has been renamed.

I note that there are a few other places where project_services gets a mention (e.g. doc/development/reactive_caching.md) but it's not immediately obvious to me that doing a search-replace is the right thing to do for all of those, so someone who knows the intent should have a look to see what needs to be done with them.

I don't think any of the rest of this template really applies to such a simple change, so I discarded it all.