Update the Refaster rule build instructions

[dmitry-timofeev]

Overview

Update the Refaster rule build instructions
to match the ones on the web site. Remove
no longer needed J--add-exports compiler
options.

See also:

• 0ede1de
• Refaster docs on the web site are out-of-date #1078
• https://errorprone.info/docs/refaster

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[dmitry-timofeev]

I signed it!

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