stty: fix typo (#6055)

According to `stty --help`, the `-F` argument must be capital (lowercase is not recognized).
Some Asian translation appears to have the same error, but I didn't want to break encoding.

pages.zh/common/stty.md

@@ -17,7 +17,7 @@
 
 - 获取设备的实际传输速度:
 
-`stty -f {{目标 / 文件夹 / 驱动设备文件}} speed`
+`stty -F {{目标 / 文件夹 / 驱动设备文件}} speed`
 
 - 将当前终端的所有模式重置为合理值:
 

pages/common/stty.md

@@ -17,7 +17,7 @@
 
 - Get the actual transfer speed of a device:
 
-`stty -f {{path/to/device_file}} speed`
+`stty -F {{path/to/device_file}} speed`
 
 - Reset all modes to reasonable values for the current terminal: