Fix 1st hello world step in the tutorial

[ledsun]

I'm confusing the Tutorial. I think some sample is old. I can run hello_world.rubex with this fix in environment below:

• ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-darwin16]
• gcc from Xcode

  gcc -v
  Configured with: --prefix=/Applications/Xcode.app/Contents/Developer/usr --with-gxx-include-dir=/usr/include/c++/4.2.1
  Apple LLVM version 9.0.0 (clang-900.0.39.2)
  Target: x86_64-apple-darwin16.7.0
  Thread model: posix
  InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

[v0dro]

Thank you. Yes the tutorial had not been updated.