[MLIR] Fix the broken format of the sparse tensor document

[Avimitin]

Summary

The documentation for the operation sparse_tensor.pack has an additional code block marker, and this breaks the following code documentation for the sparse_tensor.push_back operation.

[aartbik]

Looks like this was also done by Peiming in https://reviews.llvm.org/D151114

mlir/include/mlir/Dialect/SparseTensor/IR/SparseTensorOps.td

let description = [{
     %values      = arith.constant dense<[ 1.1,   2.2,   3.3 ]> : tensor<3xf64>
     %coordinates = arith.constant dense<[[0,0], [1,2], [1,3]]> : tensor<3x2xindex>
     %st = sparse_tensor.pack %values, %coordinates
         : tensor<3xf64>, tensor<3x2xindex> to tensor<3x4xf64, #COO>
     // yields COO format |1.1, 0.0, 0.0, 0.0|
     //     of 3x4 matrix |0.0, 0.0, 2.2, 3.3|
     //                   |0.0, 0.0, 0.0, 0.0|
     ```
-    ```
   }];
 
   let assemblyFormat =
     "$values `,` $levels attr-dict"
     "`:` type($values) `,` type($levels) `to` type($result)";
 
   let hasVerifier = 1;
 }