[3.7] bpo-32262: Fix typo in f-string (GH-7016)

Fix typo from commit 6370f345e1d5829e1fba59cd695c8b82c5a8c620

(cherry picked from commit d361e99868a9eaa7ffce9341e1207705dbb66b50)
Co-authored-by: Christian Heimes <christian@python.org>
diff --git a/Lib/asyncio/base_subprocess.py b/Lib/asyncio/base_subprocess.py
index 7c17066..b547c44 100644
--- a/Lib/asyncio/base_subprocess.py
+++ b/Lib/asyncio/base_subprocess.py
@@ -57,7 +57,7 @@
         if self._closed:
             info.append('closed')
         if self._pid is not None:
-            info.append(f'pid={self.pid}')
+            info.append(f'pid={self._pid}')
         if self._returncode is not None:
             info.append(f'returncode={self._returncode}')
         elif self._pid is not None: