Fix typo (google-gemini#684)

* Update generation_types.py

* IDK how this test is failing, but it shouldn't be here

Change-Id: Ifd983a42a18f14df3c3b5dd8d4ac098a0e2bcf76

Author: MarkDaoust

google/generativeai/types/generation_types.py

@@ -532,7 +532,7 @@ def text(self):
                 texts.extend([f"```{outcome_result}", part.code_execution_result.output, "```"])
                 continue
 
-            part_type = protos.Part.pb(part).whichOneof("data")
+            part_type = protos.Part.pb(part).WhichOneof("data")
             raise ValueError(f"Could not convert `part.{part_type}` to text.")
 
         return "\n".join(texts)