spell error: in other parts, they are lower-case

Signed-off-by: tim-zju <21651152@zju.edu.cn>
Upstream-commit: 4bf9e7caaa31035904529ee6305921821b5aa207
Component: engine

components/engine/docs/api/v1.18.md

@@ -21,8 +21,8 @@ redirect_from:
  - The daemon listens on `unix:///var/run/docker.sock` but you can
    [Bind Docker to another host/port or a Unix socket](../reference/commandline/dockerd.md#bind-docker-to-another-host-port-or-a-unix-socket).
  - The API tends to be REST, but for some complex commands, like `attach`
-   or `pull`, the HTTP connection is hijacked to transport `STDOUT`,
-   `STDIN` and `STDERR`.
+   or `pull`, the HTTP connection is hijacked to transport `stdout`,
+   `stdin` and `stderr`.
 
 ## 2. Endpoints