doc: fix double smalloc example

PR-URL: #413
Reviewed-By: Ben Noordhuis <[email protected]>

Author: mafintosh

doc/api/smalloc.markdown

@@ -44,7 +44,7 @@ possible options are listed in `smalloc.Types`. Example usage:
     var doubleArr = smalloc.alloc(3, smalloc.Types.Double);
 
     for (var i = 0; i < 3; i++)
-      doubleArr = i / 10;
+      doubleArr[i] = i / 10;
 
     // { '0': 0, '1': 0.1, '2': 0.2 }