fold-stdout: Get rid of Perl warnings about unescaped left braces

fold-stdout: Get rid of Perl warnings about unescaped left braces

Something in newer Perl versions (or maybe not too new, I haven't used
this script for years) has changed so it generated these warnings:

`$ fold-stdout out.fsck.txt`\
`Unescaped left brace in regex is passed through in regex; marked by <--
HERE in m/{{ <-- HERE {/ at /home/sunny/bin/fold-stdout line 69, <> line
1.`\
`Unescaped left brace in regex is passed through in regex; marked by <--
HERE in m/{{{ <-- HERE / at /home/sunny/bin/fold-stdout line 69, <> line
1.`

Fixed it by quoting the braces in `FOLD_BEGIN` and `FOLD_END` with
backslashes.

4b0090bc-b88c-11ee-a8c8-83850402c3ce