doc: correct sample output of buf.compare

Comparing the buffers `ABC` and `ABCD` returns `-1` not `1`.

PR-URL: #7777
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/buffer.md

@@ -806,7 +806,7 @@ console.log(buf1.compare(buf1));
 console.log(buf1.compare(buf2));
   // Prints: -1
 console.log(buf1.compare(buf3));
-  // Prints: 1
+  // Prints: -1
 console.log(buf2.compare(buf1));
   // Prints: 1
 console.log(buf2.compare(buf3));