src.ctf.fs: remove stray `}` in fmt format string

src.ctf.fs: remove stray `}` in fmt format string

Found by compiling as C++20:

    /home/smarchi/src/babeltrace/src/plugins/ctf/fs-src/query.cpp:76:9:   in 'constexpr' expansion of 'fmt::v10::basic_format_string<char, const bt2c::CStringView&>(("Cannot create metadata decoder: path=\"{}}\"."))'
    ...
     2509 |           return handler_.on_error("unmatched '}' in format string");
          |                  ~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Change-Id: I75b1429857638a00cfd18202061689710e8ea304
Signed-off-by: Simon Marchi <simon.marchi@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/12524
Tested-by: jenkins <jenkins@lttng.org>