message: fix a typo in OpDefinitions doc
  [{ matched with }], rather than ]}
  Closes
  COPYBARA_INTEGRATE_REVIEW=https://github.com/tensorflow/mlir/pull/320 from jinmingjian:patch-1
diff: @@ -505,7 +505,7 @@ def MyOp : ... {
  let builders = [
  OpBuilder<"Builder *builder, OperationState &state, float val = 0.5f", [{
  state.addAttribute("attr", builder->getF32FloatAttr(val));
  - ]}>
  + }]>
  ];
  }
  ```
  