Fix typos in programs/x509/cert_write.c

Fixes #1922.
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 09a91e0..fa99461 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -164,7 +164,7 @@
     const char *issuer_key;     /* filename of the issuer key file      */
     const char *subject_pwd;    /* password for the subject key file    */
     const char *issuer_pwd;     /* password for the issuer key file     */
-    const char *output_file;    /* where to store the constructed key file  */
+    const char *output_file;    /* where to store the constructed CRT   */
     const char *subject_name;   /* subject name for certificate         */
     const char *issuer_name;    /* issuer name for certificate          */
     const char *not_before;     /* validity period not before           */
@@ -770,7 +770,7 @@
     }
 
     /*
-     * 1.2. Writing the request
+     * 1.2. Writing the certificate
      */
     mbedtls_printf( "  . Writing the certificate..." );
     fflush( stdout );