Update advanced.md

Added comment on where the NANSAFE_MODE_ENABLED constant is located, since this was not clear.

Author: Se7enroll

docs/src/user/advanced.md

@@ -139,7 +139,7 @@ decrease performance (~5%-10% on our benchmarks).
 In order to preserve performance in the majority of use cases, ForwardDiff disables this
 check by default. If your code is affected by this `NaN` behavior, you can enable
 ForwardDiff's `NaN`-safe mode by setting the `NANSAFE_MODE_ENABLED` constant to `true` in
-ForwardDiff's source.
+ForwardDiff's source. The constant is located in `src\prelude.jl`.
 
 In the future, we plan on allowing users and downstream library authors to dynamically
 enable [NaN`-safe mode via the `AbstractConfig`