Make nodeToString brief comment clearer (NFC)

IIUC this transforms a node representation of a demangled name to the equivalent string representation. It creates the string itself, and doesn't get the string as a parameter to write into. To me "transform in" was confusing, I think the natural language to use is "transform to".

Author: danra

include/swift/Basic/Demangle.h

@@ -332,7 +332,7 @@ void mangleIdentifier(const char *data, size_t length,
 /// This should always round-trip perfectly with demangleSymbolAsNode.
 std::string mangleNode(const NodePointer &root);
 
-/// \brief Transform the node structure in a string.
+/// \brief Transform the node structure to a string.
 ///
 /// Typical usage:
 /// \code