add dot to the option of text extension

[ragingwind]

For Chrome 86 and later, without a dot, window.showSaveFilePicker cause an exception with error "Failed to execute 'showSaveFilePicker' on 'Window': Extension 'txt' must start with '.'."

I tested locally it has no problem.

[tomayac]

@petele Friendly ping to merge this. I have investigated independently and came to the same fix as in the present PR. It's literally a dot that makes the difference.

[ghost]

Seems like they ignored this PR and did it anyways: https://github.com/GoogleChromeLabs/text-editor/commit/612041a2e17696ff97ce21b51748cc4dc8feacd6

[petele]

Yep - I didn't have a CLA from this person, so I couldn't directly accept it, but I did add it in a separate PR along with a handful of other fixes... Sorry!