Fix unresolved name: insert_tail()

insert_tail(Head.next, data) --> Head.next.insert_tail(data)

Fixes: $ __flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics__
```
./data_structures/LinkedList/singly_LinkedList.py:14:13: F821 undefined name 'insert_tail'
            insert_tail(Head.next, data)
            ^
1     F821 undefined name 'insert_tail'
```

Also formats the code to be compliant with [PEP8](http://pep8.org).