Fix the display of reciprocal's formula

Author: Yibing Liu

paddle/fluid/operators/activation_op.cc

@@ -196,7 +196,7 @@ Round Activation Operator.
 __attribute__((unused)) constexpr char ReciprocalDoc[] = R"DOC(
 Reciprocal Activation Operator.
 
-$$out = \frac{1}{x}$$
+$$out = \\frac{1}{x}$$
 
 )DOC";