PR #17958: [keras/feature_column] Standardise docstring usage of "Default to"

Imported from GitHub PR #17958

This is one of many PRs. Discussion + request to split into multiple PRs @ #17748
Copybara import of the project:

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c7157c0 by Samuel Marks <[email protected]>:

[keras/feature_column/dense_features.py,keras/feature_column/sequence_feature_column.py] Standardise docstring usage of "Default to"

Merging this change closes #17958

FUTURE_COPYBARA_INTEGRATE_REVIEW=#17958 from SamuelMarks:keras.feature_column-defaults-to c7157c0
PiperOrigin-RevId: 535275621

Author: SamuelMarks

keras/feature_column/sequence_feature_column.py

@@ -122,8 +122,8 @@ def call(self, features, training=None):
             method of any `FeatureColumn` that takes a `training` argument. For
             example, if a `FeatureColumn` performed dropout, the column could
             expose a `training` argument to control whether the dropout should
-            be applied. If `None`, defaults to
-            `tf.keras.backend.learning_phase()`.
+            be applied. If `None`, becomes `tf.keras.backend.learning_phase()`.
+            Defaults to `None`.
 
 
         Returns: