Fix typo in row_event.py

Author: mjs1995

pymysqlreplication/row_event.py

@@ -642,7 +642,7 @@ def __init__(self, from_packet, event_size, table_map, ctl_connection, **kwargs)
 
                     ordinal_pos_loc += 1
                 except IndexError:
-                    # this a dirty hack to prevent row events containing columns which have been dropped prior
+                    # this is a dirty hack to prevent row events containing columns which have been dropped prior
                     # to pymysqlreplication start, but replayed from binlog from blowing up the service.
                     # TODO: this does not address the issue if the column other than the last one is dropped
                     column_schema = {