security: rename SignatureSha256 to DigestSha256

Change-Id: Idabb95222d86e2541372593698ed9754156a5df2
diff --git a/src/security/validator.cpp b/src/security/validator.cpp
index 4962779..29a4801 100644
--- a/src/security/validator.cpp
+++ b/src/security/validator.cpp
@@ -260,7 +260,7 @@
 }
 
 bool
-Validator::verifySignature(const uint8_t* buf, const size_t size, const SignatureSha256& sig)
+Validator::verifySignature(const uint8_t* buf, const size_t size, const DigestSha256& sig)
 {
   try
     {