docs: add return annotation

Signed-off-by: Athan <kgryte@gmail.com>

Author: kgryte

lib/node_modules/@stdlib/ndarray/iter/stacks/README.md

@@ -81,6 +81,7 @@ var v = iter.next().value;
 // returns <ndarray>[ [ 1, 2 ], [ 3, 4 ] ]
 
 v.set( 0, 0, 10 );
+// v => <ndarray>[ [ 10, 2 ], [ 3, 4 ] ]
 ```
 
 The returned [iterator][mdn-iterator-protocol] protocol-compliant object has the following properties: