Change 'exuberant-ctags' to 'ctags' in yum install line

-- johncburnham wrote[1] that Redhat/yum uses the package name 'ctags'

-- leaving apt install line as 'exuberant-ctags' just to be sure the
   correct one is installed.

[1] https://github.com/urbit/urbit.github.io/pull/4#issuecomment-25137418