doc: fitImage: Fix conf number incrementation

Increment conf id to avoid to have two same conf name
into one configuration description.
Signed-off-by: NMickaël Tansorier <mickael.tansorier@smile.fr>

doc/uImage.FIT/signature.txt

@@ -216,7 +216,7 @@ As an example, consider this FIT:
 			kernel = "kernel-1";
 			fdt = "fdt-1";
 		};
-		conf-1 {
+		conf-2 {
 			kernel = "kernel-2";
 			fdt = "fdt-2";
 		};
@@ -232,7 +232,7 @@ configuration 3 with kernel 1 and fdt 2:
 			kernel = "kernel-1";
 			fdt = "fdt-1";
 		};
-		conf-1 {
+		conf-2 {
 			kernel = "kernel-2";
 			fdt = "fdt-2";
 		};