[Extensions] Correct examples in runtime.onMessage description.

The description for the runtime.onMessage event incorrectly swapped the
usage examples for runtime.sendMessage and tabs.sendMessage.

Bug: 482108266
Change-Id: Ib7688fe3fabc2406dd62b183b1cb5866fa7250a6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/7567784
Reviewed-by: Tim <tjudkins@chromium.org>
Reviewed-by: Devlin Cronin <rdevlin.cronin@chromium.org>
Commit-Queue: Patrick Kettner <patrickkettner@google.com>
Cr-Commit-Position: refs/heads/main@{#1590546}
NOKEYCHECK=True
GitOrigin-RevId: c88a560110e91216fd5f35e1460ae6a305a6d4b7