Fix the latex comment syntax in sgd_op.cc (#5940)

* Fix the latex comment syntax in sgd_op.cc

* Change \textunderscore to \_

paddle/operators/sgd_op.cc

@@ -55,7 +55,7 @@ SGD operator
 
 This operator implements one step of the stochastic gradient descent algorithm.
 
-$$param_out = param - learning_rate * grad$$
+$$param\_out = param - learning\_rate * grad$$
 
 )DOC");
   }