Python: Use American English in change note

Co-authored-by: intrigus-lgtm <60750685+intrigus-lgtm@users.noreply.github.com>

Author: tausbn

python/change-notes/2021-04-13-pep249-api-graphs.md

@@ -1,4 +1,4 @@
 lgtm,codescanning
-* Modelling of libraries supporting PEP249 has been changed to use API graphs. When defining new
+* Modeling of libraries supporting PEP249 has been changed to use API graphs. When defining new
   models, the relevant extension point is now `PEP249ModuleApiNode` in the `PEP249` module, instead
   of `PEP249Module`. The latter class has now been deprecated.