Reword changelog entry to explain why it’s a bug fix.
Fix regression in `expect_any_instance_of`.

As of e00d6d7cf0f044bc9bd55d1defbbb433a88d67a4 it was
(wrongly) acting like `expect_every_instance_of`.

Fixes #699.
Merge pull request #701 from rspec/fix-699

Fix regression in `expect_any_instance_of`.