Fix readme instructions and replace stray tab indentation by spaces

[adamreisnz]

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Fix documentation referencing sgClient instead of client
• Fix indentation being tabs rather than spaces in Attachment class

[adamreisnz]

@thinkingserious just a minor PR to address two issues I noticed.

[thinkingserious]

@adamreisnz

We have not been able to merge your Pull Request, but because you are awesome - we wanted to make sure you could still get a SendGrid Hacktoberfest shirt.

Please go fill out our swag form before Nov 5th and we will send the shirt! (We know that you might have tried this before and it didn’t work, sorry about that!)

You have till Nov 5th to fill out this form in order to get the Hacktoberfest shirt!

Thank you for contributing during Hacktoberfest! We hope to see you in the repos soon! Just so you know, we always give away a SendGrid shirt for your first ever non-Hacktoberfest PR that gets merged.

[clee]

LGTM

[thinkingserious]

Hello @adamreisnz,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX