Update/fix a couple of links to fasterdata.

Motivated by #603.

.github/CONTRIBUTING.md

@@ -17,7 +17,7 @@ and/or running iperf3 on Windows, iOS, or Android.
 Before asking for help, please use your favorite search engine or
 questions site (such as Stack Overflow) to see if your question might
 have been asked (and maybe even answered) before.
-http://fasterdata.es.net/ has some information on the use of various
+https://fasterdata.es.net/ has some information on the use of various
 bandwidth measurement tools, including iperf3.  The iperf3
 documentation Web site at http://software.es.net/iperf/ contains
 various bits of helpful information, including a list of

docs/invoking.rst

@@ -6,7 +6,7 @@ The manual page is the most up-to-date reference to the various flags and parame
 
 For sample command line usage, see: 
 
-http://fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf-and-iperf3/
+https://fasterdata.es.net/performance-testing/network-troubleshooting-tools/iperf/
 
 Using the default options, iperf3 is meant to show typical well
 designed application performance.  "Typical well designed application"