Oleg’s answer allows you to disable section numbering for all exports at all times. However if you only want to disable numbering on export for a single file you can use the following:
#+OPTIONS: num:nil
If you want to limit it to only a certain depth you can replace nil with an integer specifying up to which headline you want the numbers to appear. For example #+options: num:1 would turn your sample into:
1. Heading
Sub-Heading
2. Another Heading
See Export Setting for the details and other export options that can be set on a per-file basis.
See HTML Specific Settings for HTML specific ones.