Fix typing

```
map.getkey(123)
=> nil
```

Author: ksss

test/stdlib/ObjectSpace_WeakKeyMap_test.rb

@@ -76,8 +76,8 @@ def test_getkey
     map["foo"] = 123
 
     assert_send_type(
-      "(::Integer) -> ::String",
-      map, :getkey, 123
+      "(::String) -> ::String",
+      map, :getkey, "foo"
     )
     assert_send_type(
       "(::String) -> nil",