Fix typos mostly in comments, docs and test names (GH-15209)

diff --git a/Lib/multiprocessing/util.py b/Lib/multiprocessing/util.py
index 1938091..32b51b0 100644
--- a/Lib/multiprocessing/util.py
+++ b/Lib/multiprocessing/util.py
@@ -238,7 +238,7 @@
         if self._kwargs:
             x += ', kwargs=' + str(self._kwargs)
         if self._key[0] is not None:
-            x += ', exitprority=' + str(self._key[0])
+            x += ', exitpriority=' + str(self._key[0])
         return x + '>'