Apply suggestions from code review

Co-authored-by: Sylvia Wang <139922338+sylwang@users.noreply.github.com>

Author: mjang

content/nginx-one/how-to/certificates/manage-certificates.md

@@ -75,7 +75,7 @@ For exmaple, if you use ECDSA private keys in PEM format, the PEM headers should
 
 ```
 -----BEGIN EC PRIVATE KEY-----
-<...base64-encoded key>
+<BASE64 ENCODED KEY>
 -----END EC PRIVATE KEY-----
 
 ```