doc: fix a syntax error in the buffer.md

PR-URL: https://github.com/nodejs/node/pull/9795
Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Roman Reiss <me@silverwind.io>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

doc/api/buffer.md

@@ -1122,7 +1122,7 @@ Examples:
 const buf = Buffer.from('this is a buffer');
 
 // Prints: 0
-console.log(buf.indexOf('this')));
+console.log(buf.indexOf('this'));
 
 // Prints: 2
 console.log(buf.indexOf('is'));