Markup: Update broken link in Number::toString (#3393)

spec.html

@@ -2370,7 +2370,7 @@ <h1>
             <p>Implementers of ECMAScript may find useful the paper and code written by David M. Gay for binary-to-decimal conversion of floating-point numbers:</p>
             <p>
               Gay, David M. Correctly Rounded Binary-Decimal and Decimal-Binary Conversions. Numerical Analysis, Manuscript 90-10. AT&amp;T Bell Laboratories (Murray Hill, New Jersey). 30 November 1990. Available as<br>
-              <a href="http://ampl.com/REFS/abstracts.html#rounding">http://ampl.com/REFS/abstracts.html#rounding</a>. Associated code available as<br>
+              <a href="https://ampl.com/_archive/first-website/REFS/rounding.pdf">https://ampl.com/_archive/first-website/REFS/rounding.pdf</a>. Associated code available as<br>
               <a href="http://netlib.sandia.gov/fp/dtoa.c">http://netlib.sandia.gov/fp/dtoa.c</a> and as<br>
               <a href="http://netlib.sandia.gov/fp/g_fmt.c">http://netlib.sandia.gov/fp/g_fmt.c</a> and may also be found at the various `netlib` mirror sites.
             </p>