Put variable names in backticks

Co-authored-by: Shane Frasier <[email protected]>

README.md

@@ -16,7 +16,7 @@ that can be used to create a Cyber Hygiene (CyHy) environment in AWS.
  using
  [`cisagov/cool-accounts-cyhy`](https:/cisagov/cool-accounts-cyhy).
 - A cyhy-kevsync Lambda deployment package stored in an S3 bucket (see the
- kevsync_lambda_s3_bucket and kevsync_lambda_s3_key variables).
+ `kevsync_lambda_s3_bucket` and `kevsync_lambda_s3_key` variables).
 - A valid CyHy configuration stored in the Systems Manager (SSM) Parameter
  Store of the Cyber Hygiene account (see the kevsync_lambda_config_ssm_key
  variable).