tools: bootconfig: Regex enclosed with quotes to make syntax highlight proper

As suggested, changed the square brackets escaping to quote the whole Regex
class.

Link: https://lore.kernel.org/all/20250709030141.27483-1-unixbhaskar@gmail.com/



Signed-off-by: Bhaskar Chowdhury <unixbhaskar@gmail.com>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>