make link not inline

Author: tshepang

src/solve/opaque-types.md

@@ -1,8 +1,10 @@
 # Opaque types in the new solver
 
-The way [opaque types](../opaque-types-type-alias-impl-trait.md) are handled in the new solver differs from the old implementation.
+The way [opaque types] are handled in the new solver differs from the old implementation.
 This should be a self-contained explanation of the behavior in the new solver.
 
+[opaque types]: ../opaque-types-type-alias-impl-trait.md
+
 ## opaques are alias types
 
 Opaque types are treated the same as other aliases, most notabily associated types,