Remove notice about tldr not working on Linux

I've just tried tldr out on my laptop running Ubuntu and it works!

```
$ lsb_release -a
No LSB modules are available.
Distributor ID:	Ubuntu
Description:	Ubuntu 13.04
Release:	13.04
Codename:	raring

$ tldr curl

  Transfers data from or to a server
  Supports most protocols including HTTP, FTP, POP

  - send form-encoded data

    curl --data name=bob http://localhost/form

  - send JSON data

    curl -X POST -H "Content-Type: application/json" -d '{"name":"bob"}' http://localhost/login

  - specify an HTTP method

    curl -X DELETE http://localhost/item/123

  - head request

    curl --head http://localhost
```

Let's not make Linux users sad by leaving them out of this awesome tool!  :-)