Fix spelling errors. (#992)

* extented -> extended

raster/rasterintro.html

@@ -201,7 +201,7 @@ <h3>Raster map statistics</h3>
 reports are also available (<a href="r.report.html">r.report</a>,<a
 href="r.stats.html">r.stats</a>, <a href="r.volume.html">r.volume</a>).
 
-Since <a href="r.univar.html">r.univar</a> may be slow for extented
+Since <a href="r.univar.html">r.univar</a> may be slow for extended
 statistics these can be calculated using
 <a href="r.stats.quantile.html">r.stats.quantile</a>. Without a zones input
 raster, the <a href="r.quantile.html">r.quantile</a> module will be significantly