Update g.manual.html (#207)

Add missing "s" for proper grammar.
https://trac.osgeo.org/grass/ticket/3976#comment:1

scripts/g.manual/g.manual.html

@@ -1,6 +1,6 @@
 <h2>DESCRIPTION</h2>
 
-<em>g.manual</em> display the manual pages of GRASS in HTML and MAN
+<em>g.manual</em> displays the manual pages of GRASS in HTML and MAN
 format.
 
 <h2>NOTES</h2>