Doc fix for BSL/VSL configuration

[sseago]

This makes it clear that BSL/VSL configuration in the OADP Velero CR
must specify Velero parameter names using snake_case rather than
camelCase.

Fixes #80

[sseago]

@hmuchaku and @gmerigo -- so for both of you, let me know:

1. Whether specifying s3_url in the Velero CR works for your particular use case without any further code changes
2. Whether the doc change proposed here makes it clear how to configure this.

[gmerigo]

I can confirm point 2, I cannot verify point 1 as our test cluster is currently unavailable, I waited until now to see if we could make any test but I should replicate this on another cluster, unfortunately there we don't have ebs-csi installed so the test would be very different.

[sseago]

@gmerigo Thanks for the update. I'm going to go ahead and merge this fix. If you try this change in the future and it still fails for you, feel free to open another issue.