Update default.xml

Update experimental-string-processing to use `main` from `swift/main`.

Author: compnerd

default.xml

@@ -27,7 +27,7 @@
  <project remote="github" name="swiftlang/swift-docc-render-artifact" path="swift-docc-render-artifact" />
  <project remote="github" name="swiftlang/swift-docc-symbolkit" path="swift-docc-symbolkit" />
  <project remote="github" name="swiftlang/swift-driver" path="swift-driver" />
- <project remote="github" name="swiftlang/swift-experimental-string-processing" path="swift-experimental-string-processing" revision="swift/main" />
+ <project remote="github" name="swiftlang/swift-experimental-string-processing" path="swift-experimental-string-processing" revision="main" />
  <project remote="github" name="swiftlang/swift-format" path="swift-format" />
  <project remote="github" name="swiftlang/swift-foundation" path="swift-foundation" />
  <project remote="github" name="swiftlang/swift-foundation-icu" path="swift-foundation-icu" />