Fix _find_by_data_locator

[markus-leben]

Off by one in _find_by_data_locator which causes opaque failure on data values which contain a colon.

Example html: <div data-automation-id="foo:bar">This might be trouble</div>

Example robot: Scroll Element Into View data:automation-id:foo:bar

Error message: Provided selector (automation-id:foo:bar) is malformed. Correct format: name:value.

I don't love the person putting colons in their automation ID, but we don't always get to choose the html we're testing.