Update safemath.h

Fixes dotnet#39985

src/coreclr/src/inc/safemath.h

@@ -154,8 +154,7 @@ inline bool DoubleFitsInIntType(double val)
 
  //-----------------------------------------------------------------------------
 //
-// Liberally lifted from the Office example on MSDN and modified.
-// http://msdn.microsoft.com/library/en-us/dncode/html/secure01142004.asp
+// Liberally lifted from https://github.com/dcleblanc/SafeInt and modified.
 //
 // Modified to track an overflow bit instead of throwing exceptions.  In most
 // cases the Visual C++ optimizer (Whidbey beta1 - v14.00.40607) is able to