Update the stringdoc of FactorTerm in terms.py (#286)

The stringdoc for FactorTerm specifies that only 'one-hot' coding has
been implemented but dummy has been implemented as well.

Co-authored-by: nicoleta-kyosovska <36538203+nicoleta-kyosovska@users.noreply.github.com>

Author: nicoleta-kyo

pygam/terms.py

@@ -882,9 +882,8 @@ def __init__(
 
             Custom penalties can be passed as a callable.
 
-        coding : {'one-hot'} type of contrast encoding to use.
-            currently, only 'one-hot' encoding has been developed.
-            this means that we fit one coefficient per category.
+        coding : {'one-hot', 'dummy'} type of contrast encoding to use.
+            'one-hot' keeps all columns while 'dummy' drops the first column.
 
         Attributes
         ----------