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

Mon Feb 8 09:59:56 UTC 2021

On Mon, 8 Feb 2021 07:20:05 GMT, Andrea Cosentino <github.com+5106647+oscerd 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.

Marked as reviewed by hirt (Lead).

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"?

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