README r10k example uses legacy symbol-key YAML syntax

[miharp]

The Quick Start r10k example in README.md uses legacy symbol-key YAML syntax:

:sources:
  :main:
    remote: 'git@your-git-server.example.com:puppet/control-repo.git'
    basedir: '/etc/puppetlabs/code/environments'

Verified with the current r10k: both this form and plain keys parse identically (r10k deploy display produces the same parsed sources) — r10k symbolizes keys after parsing, so the leading colons are unnecessary and just look dated:

sources:
  main:
    remote: 'git@your-git-server.example.com:puppet/control-repo.git'
    basedir: '/etc/puppetlabs/code/environments'

One-line README fix; no r10k change needed.

Raised by Yury Bushmelev (jay7x).