Update links in comments, to Laurent de Soras's exp2 approximation info

filter_variable.cpp

 		n = n << 1;
 		#else
 		// exp2 algorithm by Laurent de Soras
-		// http://www.musicdsp.org/showone.php?id=106
+		// https://www.musicdsp.org/en/latest/Other/106-fast-exp2-approximation.html
 		n = (n + 134217728) << 3;
 		n = multiply_32x32_rshift32_rounded(n, n);
 		n = multiply_32x32_rshift32_rounded(n, 715827883) << 3;

synth_waveform.cpp

 			n = n << 1;
 			#else
 			// exp2 algorithm by Laurent de Soras
-			// http://www.musicdsp.org/showone.php?id=106
+			// https://www.musicdsp.org/en/latest/Other/106-fast-exp2-approximation.html
 			n = (n + 134217728) << 3;
 			n = multiply_32x32_rshift32_rounded(n, n);
 			n = multiply_32x32_rshift32_rounded(n, 715827883) << 3;