[doc] Update index usage troubleshooting section

Contains typo fixes reported by Yaroslav Schekin in https://t.me/pgsql



git-svn-id: http://svn.osgeo.org/postgis/trunk@16601 b70326c6-7e19-0410-871a-916f4a2858ee
1 job for svn-trunk in 5 minutes and 23 seconds (queued for 3 minutes and 13 seconds)
Status Job ID Name Coverage
  Test
failed #71830125
test

00:05:23

 
Name Stage Failure
failed
test Test
  &using_postgis_dataman;
^
unable to parse postgis-out.xml
Makefile:206: recipe for target 'postgis_aggs_mm.xml' failed
make[1]: Leaving directory '/builds/postgis/postgis/doc'
make[1]: *** [postgis_aggs_mm.xml] Error 6
GNUmakefile:16: recipe for target 'all' failed
make: *** [all] Error 1
ERROR: Job failed: exit code 1