Correctly use backslashes for indent-chars

[theothornhill]

When installing the package, I get an error stating "Unmatched bracket or quote. This commit fixes it, by correctly backslashing the chararcters as perhttps://www.gnu.org/software/emacs/manual/html_node/elisp/Basic-Char-Syntax.html

']', ')' and '}' require backslash, while for the ',' it is recommended.