[mlir] Fix the example for std.rank

[herhut]

Summary

The assembly format for std.rank expects the operand type and not the
result type after the colon.

mlir/include/mlir/Dialect/StandardOps/IR/Ops.td

 def RankOp : Std_Op<"rank", [NoSideEffect]> {
   let summary = "rank operation";
   let description = [{
     The `rank` operation takes a tensor operand and returns its rank.
 
     Example:
 
     ```mlir
-    %1 = rank %0 : index
+    %1 = rank %0 : tensor<*xf32>
     ```
   }];
 
   let arguments = (ins AnyTensor);
   let results = (outs Index);
   let verifier = ?;
 
   let builders = [OpBuilder<