[jdk16] RFR: JDK-8259429: Update reference to README.md

[Erik Joelsson]

In JDK-8251551, the top level README file changed names to README.md. In jib-profiles.js we have a reference to this file to identify if the current source tree is likely to be complete. This reference needs to be updated.

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Commit messages:
 - Change reference in jib-profiles.js

Changes: https://git.openjdk.java.net/jdk16/pull/94/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=94&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259429
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/94.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/94/head:pull/94

PR: https://git.openjdk.java.net/jdk16/pull/94