Update README.md

updated ENTITY-MIB link

Author: gregbo

snmp_entity/README.md

@@ -43,5 +43,5 @@ This script has been tested with Python 2.7, and requires the following non-defa
 
 Additionally, the ENTITY-MIB needs to be loaded into net-snmp.  By default, the MIBs directory
 is `/usr/share/snmp/mibs` on Linux and `/usr/local/share/snmp/mibs` on FreeBSD.  Copy
-[ftp://ftp.cisco.com/pub/mibs/v2/ENTITY-MIB.my](ftp://ftp.cisco.com/pub/mibs/v2/ENTITY-MIB.my)
+[https://cisco.github.io/cisco-mibs/v2/ENTITY-MIB.my](https://cisco.github.io/cisco-mibs/v2/ENTITY-MIB.my)
 to that location before running this script.