docs: Remove last use of double semicolon in Makefile

Double semicolons have special meaning in makefiles, but they would have
to be combined with other rules witch such separators in order to be
used as intended.  Since there are no other rules like that, let's
clean it up.
Signed-off-by: NMartin Kletzander <mkletzan@redhat.com>

docs/Makefile.am

@@ -204,7 +204,7 @@ todo:
 	rm -f todo.html.in
 	$(MAKE) todo.html
 
-hvsupport.html:: $(srcdir)/hvsupport.html.in
+hvsupport.html: $(srcdir)/hvsupport.html.in
 
 $(srcdir)/hvsupport.html.in: $(srcdir)/hvsupport.pl $(api_DATA) \
 		$(top_srcdir)/src/libvirt_public.syms \