RFR: 7133: Update the build script help to include the installCore option

Mon Feb 8 09:59:57 UTC 2021

On Mon, 8 Feb 2021 09:00:53 GMT, Marcus Hirt <hirt at openjdk.org> wrote:

>> Noticed this: if you build jmc with build.sh script for the first time, you'll need to run the installCore command first and then all the command listed for testing and packaging it.
>
> README.md line 223:
> 
>> 221: ```
>> 222: usage: call ./build.sh with the following options:
>> 223:    --installCore to install the core bits
> 
> How about "to install the core artifacts"?

Thanks for reviewing, should be done now.

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PR: https://git.openjdk.java.net/jmc/pull/215