Increase NetCDF shared libs version number

Symbols added and removed between 4.7.0 and 4.7.3, so major bump.
OK martin@ (MAINTAINER)

math/netcdf/Makefile

@@ -1,4 +1,4 @@
-# $OpenBSD: Makefile,v 1.37 2019/12/10 17:27:20 feinerer Exp $
+# $OpenBSD: Makefile,v 1.38 2019/12/10 21:38:15 feinerer Exp $
 
 COMMENT=	library for machine-independent, array-oriented data access
 
@@ -6,10 +6,11 @@ V=		4.7.3
 NAME=		netcdf
 DISTNAME=	${NAME}-c-${V}
 PKGNAME=	${NAME}-${V}
+REVISION=	0
 
 CATEGORIES=	math
 
-SHARED_LIBS=	netcdf		5.0 # 4.0
+SHARED_LIBS=	netcdf		6.0 # 5.0
 
 HOMEPAGE=	http://www.unidata.ucar.edu/software/netcdf/