DOC Revise the math formatting for eta_0 (scikit-learn#31598)

Author: star1327p

doc/modules/sgd.rst

@@ -514,9 +514,9 @@ For regression the default learning rate schedule is inverse scaling
 
 .. math::
 
-    \eta^{(t)} = \frac{eta_0}{t^{power\_t}}
+    \eta^{(t)} = \frac{\eta_0}{t^{power\_t}}
 
-where :math:`eta_0` and :math:`power\_t` are hyperparameters chosen by the
+where :math:`\eta_0` and :math:`power\_t` are hyperparameters chosen by the
 user via ``eta0`` and ``power_t``, respectively.
 
 For a constant learning rate use ``learning_rate='constant'`` and use ``eta0``