fix: correct error message for stdout pipe check in run_process

Author: nightcityblade

newsfragments/3409.bugfix.rst

@@ -0,0 +1 @@
+Fixed incorrect error message in `run_process`: the `stdout` pipe check now correctly says "stdout" instead of "stdin".

src/trio/_subprocess.py

@@ -675,7 +675,7 @@ async def my_deliver_cancel(process):
     if task_status is trio.TASK_STATUS_IGNORED:
         if stdin is subprocess.PIPE:
             raise ValueError(
-                "stdout=subprocess.PIPE is only valid with nursery.start, "
+                "stdin=subprocess.PIPE is only valid with nursery.start, "
                 "since that's the only way to access the pipe; use nursery.start "
                 "or pass the data you want to write directly",
             )