doc: fix syntax highlighting in libswift/README.md (swiftlang#37993)

Minor documentation update to improve readability.

Author: MaxDesiatov

libswift/README.md

@@ -38,7 +38,7 @@ The core SIL C++ classes have corresponding classes in _libswift_, for example `
 
 This makes _libswift_ easy to use and it allows to program in a "Swifty" style. For example one can write
 
-```
+```swift
   for inst in block.instructions {
     if let cfi = inst as? CondFailInst {
       // ...