doc: replace undocumented encoding aliases

PR-URL: #16368
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>

Author: vsemozhetbyt

doc/api/errors.md

@@ -1224,7 +1224,7 @@ Example
 const Socket = require('net').Socket;
 const instance = new Socket();
 
-instance.setEncoding('utf-8');
+instance.setEncoding('utf8');
 ```
 
 <a id="ERR_TLS_CERT_ALTNAME_INVALID"></a>

doc/api/fs.md

@@ -1284,7 +1284,7 @@ If the file previously was shorter than `len` bytes, it is extended, and the
 extended part is filled with null bytes ('\0'). For example,
 
 ```js
-console.log(fs.readFileSync('temp.txt', 'utf-8'));
+console.log(fs.readFileSync('temp.txt', 'utf8'));
 // Prints: Node.js
 
 // get the file descriptor of the file to be truncated