rest-api-templates: Spelling fixes

Correct typos of the following word families:

overwritten
descendants

ASTERISK-29714

Change-Id: I2307e35887a3437e50317a4b86f0893f25f9fd3b

rest-api-templates/asterisk_processor.py

@@ -99,7 +99,7 @@ class PathSegment(Stringify):
             self.full_name = "%s_%s" % (parent.full_name, self.name)
 
     def get_child(self, path):
-        """Walks decendents to get path, creating it if necessary.
+        """Walks descendants to get path, creating it if necessary.
 
         @param path: List of path names.
         @return: PageSegment corresponding to path.