FIx eventHeader casing (#583)

Author: SamRemis

packages/smithy-core/src/smithy_core/traits.py

@@ -179,7 +179,7 @@ def value(self) -> str:
 
 
 @dataclass(init=False, frozen=True)
-class EventHeaderTrait(Trait, id=ShapeID("smithy.api#eventheader")):
+class EventHeaderTrait(Trait, id=ShapeID("smithy.api#eventHeader")):
     def __post_init__(self):
         assert self.document_value is None