Fix minor typo in advanced.md (#621)

* Fix minor typo in advanced.md

* Update docs/src/user/advanced.md

Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Author: thomvet

docs/src/user/advanced.md

@@ -248,8 +248,8 @@ want to disable this checking.
 
 1. (preferred) Provide an extra `Val{false}()` argument to the differentiation function, e.g.
    ```julia
-   cfg = ForwarDiff.GradientConfig(g, x)
-   ForwarDiff.gradient(f, x, cfg, Val{false}())
+   cfg = ForwardDiff.GradientConfig(g, x)
+   ForwardDiff.gradient(f, x, cfg, Val{false}())
    ```
    If using as part of a library, the tag can be checked manually via
    ```julia