Fix typo in comment.

asio/include/asio/basic_serial_port.hpp

diff --git a/asio/include/asio/basic_serial_port.hpp b/asio/include/asio/basic_serial_port.hpp
index 125565f..0f14325 100644
--- a/asio/include/asio/basic_serial_port.hpp
+++ b/asio/include/asio/basic_serial_port.hpp
@@ -428,7 +428,7 @@
    *
    * @param option The option value to be obtained from the serial port.
    *
-   * @param ec Set to indicate what error occured, if any.
+   * @param ec Set to indicate what error occurred, if any.
    *
    * @sa GettableSerialPortOption @n
    * asio::serial_port_base::baud_rate @n