[Web] Use tensor_dtype_to_np_dtype instead of deprecated function (on…

…nx#5593)

### Description
<!-- - Describe your changes. -->
Use tensor_dtype_to_np_dtype instead of deprecated
TENSOR_TYPE_TO_NP_TYPE in https://onnx.ai/onnx/intro/python.html. This
is the only place I found which mentions deprecated functions from
mapping.py.


### Motivation and Context
<!-- - Why is this change required? What problem does it solve? -->
<!-- - If it fixes an open issue, please link to the issue here. -->
Fixes onnx#5392.

---------

Signed-off-by: Chun-Wei Chen <jacky82226@gmail.com>
Co-authored-by: Justin Chu <justinchuby@users.noreply.github.com>

docs/docsgen/source/intro/python.md

@@ -148,16 +148,17 @@ of each object of the graph.
             node.name, node.op_type, node.input, node.output))
 ```
 
-The tensor type is an integer (= 1). The following array gives the
-equivalent type with numpy.
+The tensor type is an integer (= 1). The helper function {func}`onnx.helper.tensor_dtype_to_np_dtype` gives the
+corresponding type with numpy.
 
 ```{eval-rst}
 .. exec_code::
 
-    import pprint
-    from onnx.mapping import TENSOR_TYPE_TO_NP_TYPE
+    from onnx import TensorProto
+    from onnx.helper import tensor_dtype_to_np_dtype, tensor_dtype_to_string
 
-    pprint.pprint(TENSOR_TYPE_TO_NP_TYPE)
+    np_dtype = tensor_dtype_to_np_dtype(TensorProto.FLOAT)
+    print(f"The converted numpy dtype for {tensor_dtype_to_string(TensorProto.FLOAT)} is {np_dtype}.")
 ```
 
 ## Serialization