Fix a mistake in the flash method operation comment.

HirohitoHigashi · HirohitoHigashi · commit 307a4fc30256 · 2023-09-27T16:17:19.000+09:00
diff --git a/mruby_io_UART_en.md b/mruby_io_UART_en.md
@@ -182,7 +182,7 @@ flag = uart1.can_read_line()
 
 ### flush()
 
-- Returns true if reading a line of data is possible.
+- Block until transmission of data accumulated in the transmission buffer is completed.
 
 Example of use:
 
