[mlir] Fix the example for std.rank

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

Differential Revision: https://reviews.llvm.org/D79857

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

@@ -1979,7 +1979,7 @@ def RankOp : Std_Op<"rank", [NoSideEffect]> {
     Example:
 
     ```mlir
-    %1 = rank %0 : index
+    %1 = rank %0 : tensor<*xf32>
     ```
   }];