Give concept check chapter an id

doc/src/boost.xml

@@ -39,7 +39,7 @@
     <title>The Boost C++ Libraries (BoostBook Subset)</title>
     <xi:include href="libraries.xinclude" xpointer="xpointer(/*/node())"/>
 
-    <library name="Concept Check" dirname="concept_check">
+    <library name="Concept Check" dirname="concept_check" id="concept_check">
       <libraryinfo>
         <author>
           <firstname>Jeremy</firstname>