doc: fitImage: Cosmetics break lines

Add missing newline before title and subtitle.

Signed-off-by: Mickaël Tansorier <mickael.tansorier@smile.fr>
diff --git a/doc/uImage.FIT/signature.txt b/doc/uImage.FIT/signature.txt
index 1695bf4..eee0651 100644
--- a/doc/uImage.FIT/signature.txt
+++ b/doc/uImage.FIT/signature.txt
@@ -337,6 +337,7 @@
 the legacy image format is default disabled by not defining
 CONFIG_LEGACY_IMAGE_FORMAT
 
+
 Testing
 -------
 An easy way to test signing and verification is to use the test script
@@ -349,6 +350,8 @@
 $ make O=sandbox sandbox_config
 $ make O=sandbox
 $ O=sandbox ./test/vboot/vboot_test.sh
+
+
 Simple Verified Boot Test
 =========================