docs: wrap reset handler in useStopwatch example

[OFK0]

Closes #142

Problem

The README documents passing reset straight to an event handler:

<button onClick={reset}>Reset</button>

Doing that throws at runtime:
TypeError: e2.getTime is not a function at getMillisecondsFromExpiry

React invokes click handlers with the MouseEvent as the first argument, and reset only checked that its offset parameter was truthy before handing it to Time.getMillisecondsFromExpiry, which calls expiry?.getTime(). A MouseEvent is truthy and has no getTime, so it blows up.

The repo's own demo already worked around this with onClick={() => reset()}, so the README was teaching a pattern the library could not support.

Fix

Update the README stopwatch example to onClick={() => reset()}, matching the demo app.