Normalize Upcoming Feature Flag for SE-0411 (#2205)

The Upcoming Feature Flag header field typically only includes
the feature flag itself, not the compiler option.

Including the compiler option makes it more difficult for automated tools
to parse for the Swift Evolution Dashboard.

Author: dempseyatgithub

proposals/0411-isolated-default-values.md

@@ -6,7 +6,7 @@
 * Status: **Active review (October 25...November 7, 2023)**
 * Bug: *if applicable* [apple/swift#58177](https://github.com/apple/swift/issues/58177)
 * Implementation: [apple/swift#68794](https://github.com/apple/swift/pull/68794)
-* Upcoming Feature Flag: `-enable-upcoming-feature IsolatedDefaultValues`
+* Upcoming Feature Flag: `IsolatedDefaultValues`
 * Review: ([review](https://forums.swift.org/t/se-0411/68065)) ([pitch](https://forums.swift.org/t/pitch-isolated-default-value-expressions/67714))
 
 ## Introduction