doc: use sentence case for X509 error codes header

Refs: nodejs#33889

PR-URL: nodejs#41829
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Author: tniessen

doc/api/tls.md

@@ -369,7 +369,7 @@ The first three are enabled by default. The two `CCM`-based suites are supported
 by TLSv1.3 because they may be more performant on constrained systems, but they
 are not enabled by default since they offer less security.
 
-## X509 Certificate Error codes
+## X509 certificate error codes
 
 Multiple functions can fail due to certificate errors that are reported by
 OpenSSL. In such a case, the function provides an {Error} via its callback that