doc: fix a syntax error in the buffer.md

PR-URL: #9795
Reviewed-By: Sam Roberts <[email protected]>
Reviewed-By: Roman Reiss <[email protected]>
Reviewed-By: Anna Henningsen <[email protected]>

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'));