doc: fitImage: Fix conf number incrementation

Increment conf id to avoid to have two same conf name
into one configuration description.

Signed-off-by: Mickaël Tansorier <mickael.tansorier@smile.fr>
diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
index c9b1802..1695bf4 100644
--- a/doc/uImage.FIT/signature.txt
+++ b/doc/uImage.FIT/signature.txt
@@ -216,7 +216,7 @@
 			kernel = "kernel-1";
 			fdt = "fdt-1";
 		};
-		conf-1 {
+		conf-2 {
 			kernel = "kernel-2";
 			fdt = "fdt-2";
 		};
@@ -232,7 +232,7 @@
 			kernel = "kernel-1";
 			fdt = "fdt-1";
 		};
-		conf-1 {
+		conf-2 {
 			kernel = "kernel-2";
 			fdt = "fdt-2";
 		};