[NFC][Gardening] remove the invalid byte sequence example in the chan…

…gelog

"`0x7F8F`" is removed, so nobody gets the idea that there's something special about that code. "UTF-8" clarifies what encoding the byte sequence is invalid in.

Co-Authored-By: Anders Bertelrud <anders@apple.com>

CHANGELOG.md

@@ -18,7 +18,7 @@ Swift 5.3.1
 
   * Source Breakages
 
-    The package manager now throws an error if a manifest file contains invalid byte sequences such as `0x7F8F`.
+    The package manager now throws an error if a manifest file contains invalid UTF-8 byte sequences.
 
     Swift tools version specifications that contain line terminators other than `U+000A` before either "swift-tool" or "tool-version" now silently falls back to using Swift 3.1 as the lowest supported version.