swiftformat: Use language-specific heredocs

Author: issyl0

Formula/s/swiftformat.rb

@@ -31,11 +31,11 @@ def install
   end
 
   test do
-    (testpath/"potato.swift").write <<~EOS
+    (testpath/"potato.swift").write <<~SWIFT
       struct Potato {
         let baked: Bool
       }
-    EOS
+    SWIFT
     system bin/"swiftformat", "#{testpath}/potato.swift"
   end
 end