doc: fix typo in signature.txt

Fix value fields in signature nodes.

Signed-off-by: Thomas Perrot <thomas.perrot@bootlin.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
index d9a9121..7cb1c15 100644
--- a/doc/uImage.FIT/signature.txt
+++ b/doc/uImage.FIT/signature.txt
@@ -266,14 +266,14 @@
 			data = <data for fdt1>;
 			signature-1 {
 				algo = "sha1,rsa2048";
-				vaue = <...fdt signature 1...>
+				value = <...fdt signature 1...>
 			};
 		};
 		fdt-2 {
 			data = <data for fdt2>;
 			signature-1 {
 				algo = "sha1,rsa2048";
-				vaue = <...fdt signature 2...>
+				value = <...fdt signature 2...>
 			};
 		};
 	};