r.watershed: removed redundant a in the manual

git-svn-id: https://svn.osgeo.org/grass/grass/trunk@60844 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.watershed/front/r.watershed.html

@@ -114,7 +114,7 @@ their surface runoff and sedimentation yields calculated accurately.
 <p>
 Output <b>tci</b> raster map contains topographic index TCI is
 computed as
-<tt>ln(&alpha; / tan(&beta;))</tt> where &alpha; a is the cumulative
+<tt>ln(&alpha; / tan(&beta;))</tt> where &alpha; is the cumulative
 upslope area draining through a point per unit contour length and
 <tt>tan(&beta;)</tt> is the local slope angle. The TCI reflects the
 tendency of water to accumulate at any point in the catchment and the