Improve Haskell example

as proposed in https://github.com/actions/cache/issues/555

examples.md

@@ -168,8 +168,8 @@ We cache the elements of the Cabal store separately, as the entirety of `~/.caba
       ~/.cabal/packages
       ~/.cabal/store
       dist-newstyle
-    key: ${{ runner.os }}-${{ matrix.ghc }}
+    key: cabal-${{ runner.os }}-${{ matrix.ghc }}-${{ hashFiles('**/*.cabal', '**/cabal.project', '**/cabal.project.freeze') }}
-```
+    restore-keys: cabal-${{ runner.os }}-${{ matrix.ghc }}-```
 
 ## Java - Gradle