Fix error message: says stdout but checks stdin

[bysiber]

Problem

In _run_process(), when stdin=subprocess.PIPE is passed without using nursery.start, the error message incorrectly says:

stdout=subprocess.PIPE is only valid with nursery.start...

The condition checks stdin is subprocess.PIPE, but the message refers to stdout. This sends users looking at the wrong argument.

Fix

Changed "stdout=subprocess.PIPE" to "stdin=subprocess.PIPE" in the error message so it matches the actual condition being checked.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91763%. Comparing base (2fd138e) to head (3972342).

❌ Your project check has failed because the head coverage (99.91763%) is below the target coverage (100.00000%). You can increase the head coverage or adjust the target coverage.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                 main       #3398         +/-   ##
====================================================
- Coverage   100.00000%   99.91763%   -0.08237%     
====================================================
  Files             128         128                 
  Lines           19424       19424                 
  Branches         1318        1318                 
====================================================
- Hits            19424       19408         -16     
- Misses              0          12         +12     
- Partials            0           4          +4     

Files with missing lines	Coverage Δ
src/trio/_subprocess.py	96.34703% <ø> (-3.65297%)	⬇️

... and 1 file with indirect coverage changes

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[A5rocks]

Thanks! Please fix the tests and add a newsfragment.