Fix documentation generation error

Author: asterycs

docs/src/api.md

@@ -8,5 +8,5 @@ derivative(expr, wrt::DiffMatic.Monomial)
 gradient(expr, wrt::DiffMatic.Monomial)
 jacobian(expr, wrt::DiffMatic.Monomial)
 hessian(expr, wrt::DiffMatic.Monomial)
-to_std_string(expr)
+to_std(expr)
 ```