Update wheel path to Whisper custom export script (#15739)

### Description
This PR updates the documentation for using the Whisper custom export
scripts via the wheel.



### Motivation and Context
The path should say
`onnxruntime.transformers.models.whisper.convert_to_onnx` instead of
`onnxruntime.transformers.models.convert_to_onnx`.

onnxruntime/python/tools/transformers/models/whisper/README.md

@@ -26,7 +26,7 @@ Export + Optimize for FP32
 $ python3 convert_to_onnx.py -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp32
 
 # From wheel:
-$ python3 -m onnxruntime.transformers.models.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp32
+$ python3 -m onnxruntime.transformers.models.whisper.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp32
 ```
 
 Export + Optimize for FP16 and GPU
@@ -35,7 +35,7 @@ Export + Optimize for FP16 and GPU
 $ python3 convert_to_onnx.py -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp16 --use_gpu
 
 # From wheel:
-$ python3 -m onnxruntime.transformers.models.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp16 --use_gpu
+$ python3 -m onnxruntime.transformers.models.whisper.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --optimize_onnx --precision fp16 --use_gpu
 ```
 
 Export + Quantize for INT8
@@ -44,5 +44,5 @@ Export + Quantize for INT8
 $ python3 convert_to_onnx.py -m openai/whisper-tiny --output whispertiny --use_external_data_format --precision int8 --quantize_embedding_layer
 
 # From wheel:
-$ python3 -m onnxruntime.transformers.models.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --precision int8 --quantize_embedding_layer
+$ python3 -m onnxruntime.transformers.models.whisper.convert_to_onnx -m openai/whisper-tiny --output whispertiny --use_external_data_format --precision int8 --quantize_embedding_layer
 ```