[Catapult] Fix links in README.md.
[Catapult] Fix links in README.md.

Fix links so they work properly when viewed from
https://source.chromium.org/chromium/chromium/src/+/master:third_party/catapult/README.md

Change-Id: I8b8aaf7ad2fb1866b812230bc26f063447aeb17e
Reviewed-on: https://chromium-review.googlesource.com/c/catapult/+/2411479
Reviewed-by: Wenbin Zhang <45ad8a1ff616a38a2202fd35571df81897c5f14f@google.com>
Commit-Queue: John Chen <334fbfbb4df7c78f091ea1b77c69ca6ac731a3f3@chromium.org>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult