Minor fix for URI scheme whitelist demo hook

[buu700]

We just caught an issue in @cyph that was causing links sent in succession to break as in the attached screenshot, and this turned out to be the culprit.

From the MDN docs on RegExp.test: "As with exec() (or in combination with it), test() called multiple times on the same global regular expression instance will advance past the previous match."

Test for comparison:

const regex0 = /^(http|https|ftp):/gim
const regex1 = /^(http|https|ftp):/im

regex0.test('https:')
true
regex0.test('https:')
false
regex0.test('https:')
true
regex0.test('https:')
false

regex1.test('https:')
true
regex1.test('https:')
true
regex1.test('https:')
true
regex1.test('https:')
true

[cure53]

Nice find, thanks!