doc: Add links to olsen93 and ong90 in bibliography

The bibliography entries for olsen93 and ong90 lacked links to
online copies.  While ong90 is available in digital form, the
olsen93 thesis is only available as a physical copy in the UCB
library.  To save people from searching for it, we still link
to it via the UCB library page.

Reported-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/CACJufxFcJYdRvzgt59N26XjFp2tFFUXu+VN+x8Uo0NbDUCMCbw@mail.gmail.com

doc/src/sgml/biblio.sgml

@@ -286,7 +286,7 @@ ssimkovi@ag.or.at
    </biblioentry>
 
    <biblioentry id="olson93">
-    <title>Partial indexing in POSTGRES: research project</title>
+    <title><ulink url="https://search.library.berkeley.edu/permalink/01UCS_BER/iqob43/alma991082339239706532">Partial indexing in POSTGRES: research project</ulink></title>
     <authorgroup>
      <author>
       <firstname>Nels</firstname>
@@ -303,7 +303,7 @@ ssimkovi@ag.or.at
 
    <biblioentry id="ong90">
    <biblioset relation="article">
-    <title>A Unified Framework for Version Modeling Using Production Rules in a Database System</title>
+    <title><ulink url="https://www2.eecs.berkeley.edu/Pubs/TechRpts/1990/1466.html">A Unified Framework for Version Modeling Using Production Rules in a Database System</ulink></title>
     <authorgroup>
      <author>
       <firstname>L.</firstname>