docs: fix example

Author: kgryte

lib/node_modules/@stdlib/array/base/flatten5d-by/README.md

@@ -56,7 +56,7 @@ function scale( v ) {
 
 var x = [ [ [ [ [ 1, 2 ] ] ] ], [ [ [ [ 3, 4 ] ] ] ] ];
 
-var out = flatten5dBy( x, [ 2, 1, 1, 1, 2 ], true );
+var out = flatten5dBy( x, [ 2, 1, 1, 1, 2 ], true, scale );
 // returns [ 2, 6, 4, 8 ]
 ```