fix SyntaxError in coros.py

gevent/coros.py

@@ -156,9 +156,9 @@ class DummySemaphore(object):
 
 
 class BoundedSemaphore(Semaphore):
-    """A bounded semaphore checks to make sure its current value doesn’t exceed its initial value.
+    """A bounded semaphore checks to make sure its current value doesn't exceed its initial value.
     If it does, ``ValueError`` is raised. In most situations semaphores are used to guard resources
-    with limited capacity. If the semaphore is released too many times it’s a sign of a bug.
+    with limited capacity. If the semaphore is released too many times it's a sign of a bug.
 
     If not given, *value* defaults to 1."""