bpo-35133: Fix mistakes when concatenate string literals on different lines. (GH-10284)

Two kind of mistakes:

1. Missed space. After concatenating there is no space between words.

2. Missed comma. Causes unintentional concatenating in a list of strings.
diff --git a/Lib/concurrent/futures/thread.py b/Lib/concurrent/futures/thread.py
index b65dee1..7835971 100644
--- a/Lib/concurrent/futures/thread.py
+++ b/Lib/concurrent/futures/thread.py
@@ -150,7 +150,7 @@
             if self._shutdown:
                 raise RuntimeError('cannot schedule new futures after shutdown')
             if _shutdown:
-                raise RuntimeError('cannot schedule new futures after'
+                raise RuntimeError('cannot schedule new futures after '
                                    'interpreter shutdown')
 
             f = _base.Future()