Fix typos in comments and docstring

Change-Id: Id58c5ae0647894eecd3bedc4511007e2c0b9e75b

oslo_db/sqlalchemy/enginefacade.py

@@ -519,7 +519,7 @@ class _TransactionContext(object):
             session.commit()
         elif self.rollback_reader_sessions:
             session.rollback()
-        # In the absense of calling session.rollback(),
+        # In the absence of calling session.rollback(),
         # the next call is session.close().  This releases all
         # objects from the session into the detached state, and
         # releases the connection as well; the connection when returned

oslo_db/sqlalchemy/update_match.py

@@ -72,7 +72,7 @@ def update_on_match(
                 # try again
                 return False
 
-            peristent_instance = base_query.update_on_match(
+            persistent_instance = base_query.update_on_match(
                 specimen,
                 surrogate_key,
                 values=values,