Fix problem with escaped closing square brackets

"Escaping: '\' is the escape character. Any char following '\' is
inserted verbatim (exception: whitespaces still have to be converted to
space!). Following chars have to be escaped, when used in Text: ']', '\'
and ':' (only if used in compose data type)."

From: http://www.red-bean.com/sgf/sgf4.html#text

Luckily, only the closing square bracket is relevant to our parser. As
the same document states:

"When using a private value type the application has to escape every ']'
with a leading '\'. Otherwise the file would become unparseable."