active_hash: Fix the type of block argument for ActiveHash::Relation#…

…find (ruby#457)

The block should take an instance of ActiveHash::Base object, not a
relation object itself.

gems/active_hash/3.2/active_hash.rbs

@@ -68,7 +68,7 @@ module ActiveHash
             | (:first, *untyped args) -> T?
             | (Array[untyped] id) -> Array[T]
             | (untyped id) -> T?
-            | () { (instance) -> boolish } -> T?
+            | () { (T) -> boolish } -> T?
     def find_by: (untyped options) -> T?
     def find_by!: (untyped options) -> T
     def find_by_id: (untyped id) -> T?