[mlir][tensor] Fix a typo in the example code for UnrankedTensorType

[Peiming]

Summary

The syntax for unrank tensor type is defined as tensor-type ::= tensor < * x type >, the example code missed the x in between.

[aartbik]

I would use "Fix an typo in ...." in the title, but other than that, LGTM

[Peiming]

update commit message

mlir/include/mlir/IR/BuiltinTypes.td

let description = [{
 
     An unranked tensor is a type of tensor in which the set of dimensions have
     unknown rank. See [RankedTensorType](#builtin_rankedtensor-rankedtensortype)
     for more information on tensor types.
 
     Examples:
 
     ```mlir
-    tensor<*f32>
+    tensor<*xf32>
     ```
   }];
   let parameters = (ins "Type":$elementType);
 
   let builders = [
     TypeBuilderWithInferredContext<(ins "Type":$elementType), [{
       return $_get(elementType.getContext(), elementType);
     }]>