PEP 646: Fix link

Author: hugovk

pep-0646.rst

@@ -281,7 +281,7 @@ As of this PEP, only a single type variable tuple may appear in a type parameter
 
     class Array(Generic[*Ts1, *Ts2]): ...  # Error
 
-(``Union`` is the one exception to this rule; see `Type Variable Tuples with ``Union```.)
+(``Union`` is the one exception to this rule; see `Type Variable Tuples with Union`_.)
 
 Type Prefixing
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