std/bufio: fix typo

Author: mertcandav

std/bufio/bufio.jule

@@ -719,7 +719,7 @@ impl Writer {
 	// Returns an empty buffer with Reader.Available() capacity.
 	// This buffer is intended to be appended to and
 	// passed to an immediately succeeding [Writer.Write] call.
-	// The buffer is mutable and it si only valid until the next write operation.
+	// The buffer is mutable and it is only valid until the next write operation.
 	fn AvailableBuffer(mut *self): []byte {
 		ret self.buf[self.n:][:0]
 	}