Fix synthetic data generation edge case where text is much larger than requested

Author: markurtz

src/guidellm/dataset/synthetic.py

@@ -176,7 +176,7 @@ def _create_prompt(self, prompt_tokens: int, start_index: int) -> str:
             else:
                 right = mid
 
-        return self.text_creator.create_text(start_index, left)
+        return self.text_creator.create_text(start_index, left - start_index)
 
 
 class SyntheticDatasetCreator(DatasetCreator):