efi_loader: fix Sphinx warning

Brackets '[' need to be escaped to avoid a build warning

    lib/efi_loader/efi_image_loader.c:223:
    WARNING: Inline strong start-string without end-string.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Masahisa Kojima <masahisa.kojima@linaro.org>