Fixed bug in docs/airgap.js that prevented it from reading changes to the tune input

[egnima]

A textarea's value attribute gives its current, user-provided contents, while innerHTML sometimes gives its default contents. value should always be used to get the latest contents. See the specs: https://html.spec.whatwg.org/multipage/form-elements.html#dom-textarea-value

[fulldecent]

Great thank you. Merged!