Update instructions for .expected files

This phrasing bothered me when I saw it.  "ignore these files" suggests Copilot should neither write nor *read* the files, but reading them is a perfectly normal, useful thing to do.

I don't know a reasonable way to test this proposed change.

Author: geoffw0

.github/instructions/expected-files.instructions.md

@@ -1,4 +1,4 @@
 ---
 applyTo: "**/*.expected"
 ---
-Ignore these files. These are generated to be correct by running the tests, and should not be edited manually.
+Do not edit these files directly. They are generated to be correct by running the tests.