doc: replace undocumented encoding aliases

Backport-PR-URL: #16421
PR-URL: #16368
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Bryan English <bryan@bryanenglish.com>

doc/api/fs.md

@@ -954,7 +954,7 @@ 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