Update index.dd (dlang#10777)

Added std.int128 and core.int128 to the index

Author: papadil

index.dd

@@ -306,6 +306,11 @@ $(BOOKTABLE ,
         $(TDNW $(MREF std,bigint))
         $(TD An arbitrary-precision integer type.)
     )
+    $(TR
+        $(TDNW $(MREF std,int128))
+        $(TDNW $(MREF core,int128))
+        $(TD 128 bit-precision integer type.)
+    )
     $(TR
         $(TDNW $(MREF std,complex))
         $(TD A complex number type.)