docs: fix typo (#158)

Author: gdalle

docs/src/faq.md

@@ -71,7 +71,7 @@ It is important to make sure that the forward mapping and conditions accept the
 
 ```julia
 forward(x, arg1, arg2; kwarg1, kwarg2) = y
-conditions(x, arg1, arg2; kwarg1, kwarg2) = c
+conditions(x, y, arg1, arg2; kwarg1, kwarg2) = c
 ```
 
 All of the positional and keyword arguments apart from `x` will get zero tangents during differentiation of the implicit function.