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

[Iris Clark]

On Thu, 7 Jan 2021 22:01:51 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> 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.

Marked as reviewed by iris (Reviewer).

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