Put "well-formedness checking" under "lowering rules"

This was also intended to be a reference chapter, according to @scalexm.

Author: tmandry

src/SUMMARY.md

@@ -38,10 +38,10 @@
       - [Goals and clauses](./traits/goals-and-clauses.md)
       - [Equality and associated types](./traits/associated-types.md)
       - [Implied bounds](./traits/implied-bounds.md)
-      - [Well-formedness checking](./traits/wf.md)
       - [Region constraints](./traits/regions.md)
       - [The lowering module in rustc](./traits/lowering-module.md)
       - [Lowering rules](./traits/lowering-rules.md)
+      - [Well-formedness checking](./traits/wf.md)
     - [Canonical queries](./traits/canonical-queries.md)
       - [Canonicalization](./traits/canonicalization.md)
     - [The SLG solver](./traits/slg.md)