doc: corrected tlsSocket.getPeerCertificate response type

Within tls.md, the documentation for the response of
tlsSocket.getPeerCertificate() included subjectaltname
documented as an array of values. However these values are
actually returned as a single string of concatenated values.
The documentation has been updated to reflect this.

PR-URL: nodejs#27757
Fixes: nodejs#27721
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

doc/api/tls.md

@@ -917,8 +917,8 @@ certificate.
    It is returned as a `:` separated hexadecimal string. Example:
    `'2A:7A:C2:DD:...'`.
 * `ext_key_usage` {Array} (Optional) The extended key usage, a set of OIDs.
-* `subjectaltname` {Array} (Optional) An array of names for the subject, an
-   alternative to the `subject` names.
+* `subjectaltname` {string} (Optional) A string containing concatenated names
+ for the subject, an alternative to the `subject` names.
 * `infoAccess` {Array} (Optional) An array describing the AuthorityInfoAccess,
    used with OCSP.
 * `issuerCertificate` {Object} (Optional) The issuer certificate object. For