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>
diff --git a/lib/efi_loader/efi_image_loader.c b/lib/efi_loader/efi_image_loader.c
index bcd57f7..a0eb63f 100644
--- a/lib/efi_loader/efi_image_loader.c
+++ b/lib/efi_loader/efi_image_loader.c
@@ -220,7 +220,7 @@
  * @end:	End address of region (excluded)
  * @nocheck:	flag against overlapped regions
  *
- * Take one entry of region [@start, @end[ and insert it into the list.
+ * Take one entry of region \[@start, @end\[ and insert it into the list.
  *
  * * If @nocheck is false, the list will be sorted ascending by address.
  *   Overlapping entries will not be allowed.