spl: correct link to FIT specification

Replace the invalid link to the FIT file format specification.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/common/spl/spl_fit.c b/common/spl/spl_fit.c
index 988125b..2a097f4 100644
--- a/common/spl/spl_fit.c
+++ b/common/spl/spl_fit.c
@@ -587,7 +587,7 @@
 static void warn_deprecated(const char *msg)
 {
 	printf("DEPRECATED: %s\n", msg);
-	printf("\tSee doc/uImage.FIT/source_file_format.txt\n");
+	printf("\tSee https://fitspec.osfw.foundation/\n");
 }
 
 static int spl_fit_upload_fpga(struct spl_fit_info *ctx, int node,