Avoid extra whitespace in the arguments of <indexterm>.

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.