Fix a typo.

Lib/test/test_subprocess.py

@@ -471,7 +471,7 @@ class ProcessTestCase(BaseTestCase):
         self.assertStderrEqual(stderr.encode(), b"pineapple\npear\n")
 
     def test_communicate_timeout_large_ouput(self):
-        # Test a expring timeout while the child is outputting lots of data.
+        # Test an expiring timeout while the child is outputting lots of data.
         p = subprocess.Popen([sys.executable, "-c",
                               'import sys,os,time;'
                               'sys.stdout.write("a" * (64 * 1024));'