Fixed assorted typos (Collector #218)

src/BTrees/Length.py

@@ -86,7 +86,7 @@
 import Persistence
 
 class Length(Persistence.Persistent):
-    """BTree lengths aqre too expensive to compute
+    """BTree lengths are too expensive to compute
 
     Objects that use BTrees need to keep track of lengths themselves.
     This class provides an object for doing this.

src/BTrees/convert.py

@@ -84,7 +84,7 @@
 ##############################################################################
 
 def convert(old, new, threshold=200, f=None, None=None):
-    "Utility for converting old btree new new"
+    "Utility for converting old btree to new"
     n=0
     for k, v in old.items():
         if f is not None: v=f(v)