r.relief: update parameter name in man page (merge from trunk, https://trac.osgeo.org/grass/changeset/64543)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@64544 15284696-431f-4ddb-bdfa-cd5b030d7da7

raster/r.relief/r.relief.html

@@ -24,7 +24,7 @@ number, then the ewres and nsres are multiplied by that scale to
 calculate the shading. (Default=1.0 for equivalent horizontal and 
 vertical scales.)
 
-<li>The <b>zmult</b> exaggeration factor that changes the apparent relief
+<li>The <b>zscale</b> exaggeration factor that changes the apparent relief
 for the shaded relief map.  This can be any positive (or negative) floating
 point value. (Default=1.0)