doc: Fix case in titles

Unified case in titles.
Fixed small grammar issues.

Signed-off-by: Francesco Servidio <francesco.servidio@nordicsemi.no>
diff --git a/docs/signed_images.md b/docs/signed_images.md
index 22a6836..c1edcfb 100644
--- a/docs/signed_images.md
+++ b/docs/signed_images.md
@@ -91,7 +91,7 @@
 
     const int bootutil_key_cnt = sizeof(bootutil_keys) / sizeof(bootutil_keys[0]);
 
-## Building bootloader
+## Building the bootloader
 
 Enable the BOOTUTIL_SIGN_RSA syscfg setting in your app or target syscfg.yml
 file