Fix canister call example in Quick start

[zippoxer]

I haven't even finished reading the Quick start and already found what seems like a (broken) legacy example. I quickly found the solution though, and would ❤ to share it!

Problem
Executing this snippet from the Quick start:

> dfx canister call hello greet everyone

Gives:

Invalid argument: Invalid IDL: IDL parser error: Unrecognized token `Id("everyone")` found at 0:8

Why it happens
When executing the dfx canister call command, the default argument type (--type) is idl which is incompatible with the example which tries to pass a string argument without saying it's a string.

I guess the default used to be string in the past?

Solution
This PR appends --type=string to the broken command, and now it's fixed!

[lsgunnlsgunn]

Is is a good catch--our doc escaped from the Zoo a little early! Your fix works for 0.5.7 and earlier, but the --type string setting has been deprecated for the next release (which isn't publicly available just yet).

[lsgunnlsgunn]

Thanks again for your contribution. We hope to have the doc and code back in synch this week, so I'm closing this PR, but please do retest when the latest version is available for download and let us know if you find anything amiss.