paasio: Title as "PaaS I/O" instead of default Paasio (#878)

To match the website listing it as "Paas I/O":
http://exercism.io/tracks/rust/exercises/paasio

The default was "Paasio" and the README still contains this.
Change it to match the website.

Given that the exercise is about input/output statistics for a
platform-as-a-service, "PaaS I/O" seems a better way than the default.

exercism/problem-specifications#1589

exercises/paasio/README.md

@@ -1,4 +1,4 @@
-# Paasio
+# PaaS I/O
 
 Report network IO statistics.