[doc] Mention 'brew link libomp' in macOS setup guide (#696)

Add missing bullet in macOS set up guide that could be helpful on some
systems

Author: aobolensk

docs/user_guide/environment.rst

@@ -125,6 +125,7 @@ Parallel Programming Technologies
 
      brew install llvm
      brew install libomp
+     brew link libomp --overwrite --force
 
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