[libstdc++,doc] adjust a link in doc/xml/manual/allocator.xml

[Gerald Pfeifer]

This just moved from http to https, and they put a redirect in place,
so pretty straightforward.  Applied.

2018-10-01  Gerald Pfeifer  <gerald@pfeifer.com>

	* doc/xml/manual/allocator.xml: Adjust link to "Reconsidering
	Custom Memory Allocation".

Index: doc/xml/manual/allocator.xml
===================================================================
--- doc/xml/manual/allocator.xml	(revision 264760)
+++ doc/xml/manual/allocator.xml	(revision 264761)
@@ -531,7 +531,7 @@
   <biblioentry>
       <title>
 	<link xmlns:xlink="http://www.w3.org/1999/xlink"
-	      xlink:href="http://people.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">
+	      xlink:href="https://people.cs.umass.edu/~emery/pubs/berger-oopsla2002.pdf">
       Reconsidering Custom Memory Allocation
 	</link>
       </title>