r.horizon manual - fix typo (#2794)

Author: hellik

raster/r.horizon/r.horizon.html

@@ -125,7 +125,7 @@ <h2>METHOD</h2>
 This is continued until the line of sight reaches a height that is
 higher than any point in the region or until it reaches the border of
 the region (see also the <i>bufferzone,e_buff</i>, <i>n_buff</i>,
-<i>s_buff</i>, and <i>w_buff</i>). The the number of lines of sight (azimuth
+<i>s_buff</i>, and <i>w_buff</i>). The number of lines of sight (azimuth
 directions) is determined from the <i>direction</i> and
 <i>step</i> parameters. The method takes into account the curvature
 of the Earth whereby remote features will seem to be lower than they