fixup! bigfile/zodb: Add ZBlk format option 'auto' (heuristic)

Fix typo.

bigfile/file_zodb.py

@@ -613,7 +613,7 @@ class ZBigFile(LivePersistent):
         #        - for current block use ZBlk1 if the append is small and not fully filled, and ZBlk0 otherwise
         #
         # do the migration of previous block only if it is also changed in
-        # current transaction. This preserves the invairant that "transaction
+        # current transaction. This preserves the invariant that "transaction
         # changes ZBlk objects only for modified blocks of the file".
         # NOTE: this misses a case when append stops exactly on blocks boundary
         # after appending some zeros. For now we ignore such case as improbable.