Fixes a bug that causes IPv6 addresses to get the wrong port.

Bug: None
Test: UT
Change-Id: I6f49b735e74d94c8be61814072ce106800e6ded6
diff --git a/acts/framework/acts/controllers/iperf_server.py b/acts/framework/acts/controllers/iperf_server.py
index d2ac30b..260fd2b 100755
--- a/acts/framework/acts/controllers/iperf_server.py
+++ b/acts/framework/acts/controllers/iperf_server.py
@@ -335,7 +335,7 @@
         if pid in line:
             # Expected format:
             # tcp LISTEN  0 5 *:<PORT>  *:* users:(("cmd",pid=<PID>,fd=3))
-            return line.split()[4].split(':')[1]
+            return line.split()[4].split(':')[-1]
     else:
         raise ProcessLookupError('Could not find started iperf3 process.')