Fix a typo and remove -z flag mention in v.out.ogr warning messages (#1694)

* Fix a typo in v.out.ogr warning message
* Remove -z flag mention in v.out.ogr warning

The -z flag was introduced in GRASS 6.4 with commit https://github.com/OSGeo/grass/commit/ffaa43069530312280cd007228427834eabe8812 and removed in GRASS 7.0 with commit https://github.com/OSGeo/grass/commit/606b979cabf0aeae15ad718deec93caecab44f3b

vector/v.out.ogr/main.c

@@ -666,7 +666,7 @@ int main(int argc, char *argv[])
 	    if (!shpt || shpt[strlen(shpt) - 1] != 'Z') {
 		G_warning(_("Vector map <%s> is 3D. "
 			    "Use format specific layer creation options SHPT (parameter 'lco') "
-			    "or '-z' flag to export in 3D rather than 2D (default)"),
+			    "to export in 3D rather than 2D (default)"),
 			  options.input->answer);
 	    }
 	}