Add a cabal-spec changelog entry about SPDX license list 3.2

Correct previous markup [skip ci]

Cabal/doc/file-format-changelog.rst

@@ -20,3 +20,6 @@ Changes in 2.4
     the candidate file's extension, rather than requiring strict
     equality (e.g., previously ``*.html`` did not match
     ``foo.en.html``, but now it does).
+
+* License fields use identifiers from SPDX License List version
+  ``3.2 2018-07-10``