doc: Fixed notes formatting

Fixed the style used to format notes.

Signed-off-by: Francesco Servidio <francesco.servidio@nordicsemi.no>
diff --git a/docs/encrypted_images.md b/docs/encrypted_images.md
index e78713a..6ce52f1 100644
--- a/docs/encrypted_images.md
+++ b/docs/encrypted_images.md
@@ -132,8 +132,13 @@
 sectors are re-encrypted when copying from the `primary slot` to
 the `secondary slot`.
 
-PS: Each encrypted image must have its own key TLV that should be unique
-and used only for this particular image.
+---
+***Note***
+
+*Each encrypted image must have its own key TLV that should be unique*
+*and used only for this particular image.*
+
+---
 
 Also when swap method is employed, the sizes of both images are saved to
 the status area just before starting the upgrade process, because it