Update cloudhub-architecture.adoc

[ashishapisero]

More description of what worker memory and heap memory means.

Writer's Quality Checklist

Before merging your PR, did you:

• Run spell checker
• Run link checker to check for broken xrefs
• Check for orphan files
• Perform a local build and do a final visual check of your content, including checking for:
  • Broken images
  • Dead links
  • Correct rendering of partials if they are used in your content
  • Formatting issues, such as:
    • Misnumbered ordered lists (steps) or incorrectly nested unordered lists
    • Messed up tables
    • Proper indentation
    • Correct header levels
• Receive final review and signoff from:
  • Technical SME
  • Product Manager
  • Editor or peer reviewer
  • Reporter, if this content is in response to a reported issue (internal or external feedback)
• If applicable, verify that the software actually got released

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @ashishapisero to sign the Salesforce.com Contributor License Agreement.