fix typos

src/BTrees/Length.py

@@ -15,7 +15,7 @@
 import persistent
 
 class Length(persistent.Persistent):
-    """BTree lengths are too expensive to compute
+    """BTree lengths are often too expensive to compute.
 
     Objects that use BTrees need to keep track of lengths themselves.
     This class provides an object for doing this.
@@ -27,7 +27,7 @@ class Length(persistent.Persistent):
     to provide instance-specific __len__ methods.  However, this no
     longer works as expected, because new-style classes cache
     class-defined slot methods (like __len__) in C type slots.  Thus,
-    instance-define slot fillers are ignores.
+    instance-defined slot fillers are ignored.
 
     """