Fix broken link in the docstring of Thunk (#560)

* Fix broken link in the docstring of `Thunk`

* Update Project.toml

Author: devmotion

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.15.0"
+version = "1.15.1"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/tangent_types/thunks.jl

@@ -186,7 +186,7 @@ with a field for each variable used in the expression, and call overloaded.
 Do not use `@thunk` if this would be equal or more work than actually evaluating the expression itself.
 This is commonly the case for scalar operators.
 
-For more details see the manual section [on using thunks effectively](http://www.juliadiff.org/ChainRulesCore.jl/dev/writing_good_rules.html#Use-Thunks-appropriately-1)
+For more details see the manual section [on using thunks effectively](https://juliadiff.org/ChainRulesCore.jl/dev/rule_author/writing_good_rules.html#Use-Thunks-appropriately).
 """
 struct Thunk{F} <: AbstractThunk
     f::F