shared/runtime/sys_stdio_mphal: Fix docstring for stdio.

Signed-off-by: timdechant <timdechant.git@gmail.com>

Author: timdechant

tests/basics/sys_stdio.py

@@ -2,6 +2,14 @@
 
 import sys
 
+try:
+    sys.stdout
+    sys.stdin
+    sys.stderr
+except AttributeError:
+    print("SKIP")
+    raise SystemExit
+
 # CPython is more verbose; no need to match exactly
 
 print('TextIOWrapper' in str(sys.stdout))