message: fix broken link to optional metadata in IR.md
diff: # Metadata
  # Metadata
  -In addition to the core metadata recommendations listed in the [extensibility documentation](IR.md#metadata) there is additional experimental metadata to help provide information for model inputs and outputs.
  +In addition to the core metadata recommendations listed in the [extensibility documentation](IR.md#optional-metadata) there is additional experimental metadata to help provide information for model inputs and outputs.
  This metadata applies to all input and output tensors of a given category. The first such category we define is: `Image`.
  