message: Fix typo in mapping.rst
  correspondance -> correspondence
diff: onnx.mapping
  ============
  -This module defines the correspondance between onnx numerical types
  +This module defines the correspondence between onnx numerical types
  and numpy numerical types. This information can be accessed
  through attribute :ref:`l-onnx-types-mapping` or through the functions
  defined in :ref:`l-mod-onnx-helper`.
  