Update documentation to reflect GRASS_TRUECOLOR=TRUE by default

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@33123 15284696-431f-4ddb-bdfa-cd5b030d7da7

lib/pngdriver/pngdriver.html

@@ -40,7 +40,7 @@ of the PNG driver.
   <LI><B>GRASS_TRANSPARENT=[TRUE|FALSE]</B><BR>
     sets transparent background on (TRUE) or off (FALSE, default).</LI>
   <LI><B>GRASS_TRUECOLOR=[TRUE|FALSE]</B><BR>
-    sets true-color support</LI>
+    sets true-color support. Default is TRUE</LI>
   <LI><B>GRASS_PNG_COMPRESSION=[0|1|9]</B><BR>
     compression level of PNG files (0 = none, 1 = fastest, 9 = best, default is 6)</LI>
   <LI><B>GRASS_PNG_AUTO_WRITE=[TRUE|FALSE]</B><BR>