fix charlist warning

[dnsbty]

Compiling the project produces this warning for using a single quoted string to produce a charlist:

    warning: using single-quoted strings to represent charlists is deprecated.
    Use ~c"" if you indeed want a charlist or use "" instead.
    You may run "mix format --migrate" to change all single-quoted
    strings to use the ~c sigil and fix this warning.
    │
 16 │     path = :filename.join(:code.priv_dir(unquote(app)), 'sippet_nif')
    │                                                         ~
    │
    └─ lib/sippet/parser.ex:16:57

This change updates the code to use the ~c sigil to fix the warning.