Fix Elixir 1.17+ warning (Single-quote charlists are deprecated, use ~c instead)

[thbar]

Description

Compilation (before this PR) of the library generates a lot of warnings on Elixir 1.17+:

     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.
     │
 211 │     '}'
     │     ~
     │
     └─ lib/encode.ex:211:5

Motivation and Context

The goal is to see clearer in remaining warnings & test failures on Elixir 1.18+, as I'm using the library.

Types of changes

I ran mix format --migrate on Elixir 1.18, and kept only the ~c migration relevant bits.