RFR: JDK-8266227: Fix help text for --mac-signing-keychain

[Andy Herrick]

On Fri, 30 Apr 2021 15:25:13 GMT, Andy Herrick <herrick at openjdk.org> wrote:

> JDK-8266227: Fix help text for --mac-signing-keychain

I updated the bug Description to point out that text formatting and other minor wording changes are being applied to the help

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PR: https://git.openjdk.java.net/jdk/pull/3819