Target the IsList instance for ZipList at base-4.14.0.0 (#17489)

This moves the changelog entry about the instance from
`base-4.15.0.0` to `base-4.14.0.0`. This accomplishes part (1)
from #17489.

[ci skip]

libraries/base/changelog.md

 # Changelog for [`base` package](http://hackage.haskell.org/package/base)
 
 ## 4.15.0.0 *TBA*
-  * Add `IsList` instance for `ZipList`.
+  * TODO
 
 ## 4.14.0.0 *TBA*
   * Bundled with GHC 8.10.1
@@ -46,6 +46,8 @@
 
   * Add a `Data` instance to `WrappedArrow`, `WrappedMonad`, and `ZipList`.
 
+  * Add `IsList` instance for `ZipList`.
+
 ## 4.13.0.0 *July 2019*
   * Bundled with GHC 8.8.1