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

https://github.com/exercism/problem-specifications/pull/1589
petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust,petertseng/exercism-rust