Fix comments about filenames in some programs
diff --git a/programs/pkey/rsa_sign_pss.c b/programs/pkey/rsa_sign_pss.c
index 0b3cfbb..dd9386b 100644
--- a/programs/pkey/rsa_sign_pss.c
+++ b/programs/pkey/rsa_sign_pss.c
@@ -140,7 +140,7 @@
     }
 
     /*
-     * Write the signature into <filename>-sig.txt
+     * Write the signature into <filename>.sig
      */
     mbedtls_snprintf( filename, 512, "%s.sig", argv[2] );