Fix typo

tools/i-pi/ipi/utils/io/io_xml.py

@@ -119,7 +119,7 @@ class xml_handler(ContentHandler):
 
       Adds the opening tag to the list of open tags, adds a new space in the
       buffer, reads the appropriate attributes and adds a new level to the 
-      heirarchy.
+      hierarchy.
 
       Args:
          name: The tag_name.
@@ -139,7 +139,7 @@ class xml_handler(ContentHandler):
    def characters(self, data):
       """Reads data.
 
-      Adds the data to the buffer of the current level of the heirarchy.
+      Adds the data to the buffer of the current level of the hierarchy.
       Data is read as a string, and needs to be converted to the required
       type later.