Fix URL in docstring

Author: Cito

src/graphql/validation/rules/overlapping_fields_can_be_merged.py

@@ -50,7 +50,7 @@ class OverlappingFieldsCanBeMergedRule(ValidationRule):
     A selection set is only valid if all fields (including spreading any fragments)
     either correspond to distinct response names or can be merged without ambiguity.
 
-    See https://spec.graphql.org/draft/#sec-Field-Selection-Mergin
+    See https://spec.graphql.org/draft/#sec-Field-Selection-Merging
     """
 
     def __init__(self, context: ValidationContext):