Fix `paddle.atan` docstring (#25968)

test=develop,test=document_fix

remove activation wording
tanh -> tan

paddle/fluid/operators/activation_op.cc

@@ -365,9 +365,9 @@ class AtanOpMaker : public framework::OpProtoAndCheckerMaker {
     AddInput("X", "Input of atan operator");
     AddOutput("Out", "Output of atan operator");
     AddComment(R"DOC(
-Arctanh Activation Operator.
+Arctangent Operator.
 
-$$out = \tanh^{-1}(x)$$
+$$out = \tan^{-1}(x)$$
 
 )DOC");
   }