RFR: Update Readme with macOs specifics [v2]

Tue Mar 29 10:33:00 UTC 2022

On Tue, 29 Mar 2022 10:10:33 GMT, Brice Dutheil <duke at openjdk.java.net> wrote:

>> Brice Dutheil has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Fix review comments
>
> README.md line 54:
> 
>> 52: 
>> 53: ```sh
>> 54: $ sh ./gradlew -Pjdk18_home=<jdk18_home_dir> -Pllvm_home=<libclang_dir> -Pjtreg_home=<jtreg_home> jtreg
> 
> I noticed `jtreg` task also required this property to be set.
> 
> However I do have a question about which version of jtreg should be used, as a convenience I downloaded jtreg6 from Aleksey Shipilëv https://builds.shipilev.net/jtreg/

that version of jtreg should be ok

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PR: https://git.openjdk.java.net/jextract/pull/10