README.md: Fix typo constained → constrain (#49)

Author: schwehr

README.md

@@ -42,7 +42,7 @@ conda install -c conda-forge traittypes
 
 `traittypes` extends the `traitlets` library with an implementation of trait types for numpy arrays, pandas dataframes, pandas series, xarray datasets and xarray dataarrays.
  - `traittypes` works around some limitations with numpy array comparison to only trigger change events when necessary.
- - `traittypes` also extends the traitlets API for adding custom validators to constained proposed values for the attribute.
+ - `traittypes` also extends the traitlets API for adding custom validators to constrain proposed values for the attribute.
 
 For a general introduction to `traitlets`, check out the [traitlets documentation](https://traitlets.readthedocs.io/en/stable/).