Fix PoshChan settings to use dashes instead of underscore

[SteveL-MSFT]

PR Summary

VSO context names for this repo uses dashes, but the config was using underscore. Fix is to use dashes.

PR Context

PR Checklist

• PR has a meaningful title
  • Use the present tense and imperative mood when describing your changes
• Summarized changes
• Change is not breaking
• Make sure all .h, .cpp, .cs, .ps1 and .psm1 files have the correct copyright header
• This PR is ready to merge and is not Work in Progress.
  • If the PR is work in progress, please add the prefix WIP: or [ WIP ] to the beginning of the title (the WIP bot will keep its status check at Pending while the prefix is present) and remove the prefix when the PR is ready.
• User-facing changes
  • Not Applicable
  • OR
  • Documentation needed
    • Issue filed:
• Testing - New and feature
  • N/A or can only be tested interactively
  • OR
  • Make sure you've added a new test if existing tests do not effectively test the code changed
    • Add [feature] to your commit messages if the change is significant or affects feature tests

[SteveL-MSFT]

@adityapatwardhan can you merge? Thanks!