RFR: 8373117: Update build doc link in README.md

[Erik Joelsson]

The main README.md in the JDK repository has a link to https://openjdk.org/groups/build/doc/building.html. This page is being retired in favor of the markdown file in GitHub, so we should update the link to point directly to https://git.openjdk.org/jdk/blob/master/doc/building.md.

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Commit messages:
 - JDK-8373117

Changes: https://git.openjdk.org/jdk/pull/28701/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28701&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373117
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/28701.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28701/head:pull/28701

PR: https://git.openjdk.org/jdk/pull/28701