Update SXG version in wpt/signed-exchange/README.md

We already use version b3 in the tests.

Change-Id: Iadcf6b0a1057d6af01884f524ece0394bf244625
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1824499
Commit-Queue: Kunihiko Sakamoto <ksakamoto@chromium.org>
Commit-Queue: Tsuyoshi Horo <horo@chromium.org>
Auto-Submit: Kunihiko Sakamoto <ksakamoto@chromium.org>
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#699638}

third_party/blink/web_tests/external/wpt/signed-exchange/README.md

 This directory contains tests for the
-[b2 implementation snapshot](https://tools.ietf.org/html/draft-yasskin-httpbis-origin-signed-exchanges-impl-02) of the
+[b3 implementation snapshot](https://tools.ietf.org/html/draft-yasskin-httpbis-origin-signed-exchanges-impl-03) of the
 [Signed HTTP Exchanges](https://wicg.github.io/webpackage/draft-yasskin-http-origin-signed-responses.html).
 
 `resources` directory contains pregenerated signed exchanges and