Fix man page

`nodetree -h` outputs `man nodetree` to `stdout` without showing the man page.

#### Environment
```
OS X 10.13.4
zsh 5.4.2 (x86_64-apple-darwin16.7.0)
GNU bash, version 4.4.12(1)-release (x86_64-apple-darwin16.3.0)
nodetree 0.0.3
```