Typo on buffer[_size]

Bag-of-Tricks.md

@@ -104,7 +104,7 @@ class BufferedPrint : public Print {
   }
 
   void flush() {
-    buffer[_size] = '\0';
+    _buffer[_size] = '\0';
     _destination.print(_buffer);
     _size = 0;
   }