Add 'id' to Acknowledgments section

Per note from Erik Rijkers
Discussion: https://postgr.es/m/3db724af16ee009ab7f812a6a1d9354e@xs4all.nl

Author: alvherre

doc/src/sgml/release-10.sgml

@@ -8990,7 +8990,7 @@ This was disabled in the PG 9.6 branch so there is no commit here.
 
   </sect2>
 
-  <sect2>
+  <sect2 id="release-10-acknowledgements">
    <title>Acknowledgments</title>
 
    <para>

doc/src/sgml/release-11.sgml

@@ -3633,7 +3633,7 @@ same commits as above
 
   </sect2>
 
-  <sect2>
+  <sect2 id="release-11-acknowledgements">
    <title>Acknowledgments</title>
 
    <para>