Remove 'sub-section' from source code

[alexeymalihin]

Checking if h3 is not is source_example.
Prevents adding sub navigation link if h3 is part of source code.

[robhrt7]

Hi,

Thank you for noticing quite major bug. But lets do it simpler, just like that:

var subHeaders = section.children("h3");

Because we need only .section > h3

[alexeymalihin]

Hi,

We couldn't have sub-sub-section? Like .section > h3 > h3? If no, than decline PR and fix it as you suggested please.

[robhrt7]

We treat h4 as sub-section of h3, and all tags are on one level of HTML nesting. We will fix this issue in upcoming 0.4.1.

[robhrt7]

45ea069