package/snmppp: fix broken URL in help text

The curren URL is specific to an old version, and it is not reachable
anymore.

Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/snmppp/Config.in

@@ -6,7 +6,7 @@ config BR2_PACKAGE_SNMPPP
 	help
 	  SNMP++v3.x is a C++ API which supports SNMP v1, v2c, and v3.
 
-	  http://www.agentpp.com/snmp_pp3_x/snmp_pp3_x.html
+	  https://www.agentpp.com/api/cpp/snmp_pp.html
 
 if BR2_PACKAGE_SNMPPP