Fix error message: says stdout but checks stdin

The ValueError raised when stdin=subprocess.PIPE is used without
nursery.start incorrectly says 'stdout=subprocess.PIPE'.  This is
confusing for users since the actual problem is with the stdin
argument, not stdout.

Author: bysiber

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",
             )