Land rapid7#8893, update meterpreter repo link

Brent Cook · Brent Cook · commit 406cbd09853b · 2017-08-27T21:31:34.000-05:00
diff --git a/external/source/meterpreter/README b/external/source/meterpreter/README
@@ -1,2 +1,2 @@
-Meterpreter source code has moved to its own repository, hosted at
-https://github.com/rapid7/meterpreter
+Meterpreter source code is part of the metasploit-payloads repository, hosted at
+https://github.com/rapid7/metasploit-payloads
