Use posix compliant tail syntax

homebrew allows using gnu coreutils on MacOS. Therefore we should
comply with posix standards

```bash
$ mas help | tail +3
tail: cannot open '+3' for reading: No such file or directory
$ type tail
tail is /usr/local/opt/coreutils/libexec/gnubin/tail
```

Switching to posix compliant `-n` syntax is compatible with UNIX and GNU tooling.

```bash
$ mas help | /usr/bin/tail -n +3 | wc -l
18
$ mas help | tail -n +3 | wc -l
18
```

contrib/completion/mas-completion.bash

@@ -9,7 +9,7 @@ _mas() {
     _get_comp_words_by_ref cur prev words cword
   fi
   if [[ $cword -eq 1 ]]; then
-    COMPREPLY=($(compgen -W "$(mas help | tail +3 | awk '{print $1}')" -- "$cur"))
+    COMPREPLY=($(compgen -W "$(mas help | tail -n +3 | awk '{print $1}')" -- "$cur"))
     return 0
   fi
 }