Avoid extra whitespace in the arguments of <indexterm>.

As noted by Thom Brown, this confuses the DocBook index processor; it
fails to merge entries that differ only in whitespace, and sorts them
unexpectedly as well.  Seems like a toolchain bug, but I'm not going to
hold my breath waiting for a fix.

Note: easiest way to find these is to look for double spaces in HTML.index.

doc/src/sgml/pltcl.sgml

@@ -344,8 +344,8 @@ spi_exec -array C "SELECT * FROM pg_class" {
        <para>
         Prepares and saves a query plan for later execution.  The
         saved plan will be retained for the life of the current
-        session.<indexterm><primary>preparing a query</><secondary>in
-        PL/Tcl</></>
+        session.<indexterm><primary>preparing a query</>
+        <secondary>in PL/Tcl</></>
        </para>
        <para>
         The query can use parameters, that is, placeholders for