doc: replace methods used in the example code

Methods `buf.writeUIntLE()` and `buf.writeUIntBE()` were used
in the example code for the section of the methods
`writeIntLE()` and `writeIntBE()` instead of the latter.

PR-URL: nodejs/node#16416
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>

doc/api/buffer.md

@@ -2383,12 +2383,12 @@ Examples:
 ```js
 const buf = Buffer.allocUnsafe(6);
 
-buf.writeUIntBE(0x1234567890ab, 0, 6);
+buf.writeIntBE(0x1234567890ab, 0, 6);
 
 // Prints: <Buffer 12 34 56 78 90 ab>
 console.log(buf);
 
-buf.writeUIntLE(0x1234567890ab, 0, 6);
+buf.writeIntLE(0x1234567890ab, 0, 6);
 
 // Prints: <Buffer ab 90 78 56 34 12>
 console.log(buf);