Update Spark Job Namespace docs

[matthewrossi]

Purpose of this PR

In #1955 the use of sparkJobNamespace was replaced with sparkJobNamespaces. However the README.md and the Quick Start Guide still show the use of the old Helm value.

Proposed changes:

• Update docs with the new Helm value

Change Category

Indicate the type of change by marking the applicable boxes:

• Bugfix (non-breaking change which fixes an issue)
• Feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that could affect existing functionality)
• Documentation update

Additional Notes

• To the best of my knowledge there are two ways to set a list value of a Helm Chart: --set "sparkJobNamespaces={default}" and --set sparkJobNamespaces[0]=default (for personal preference I went with the first one)
• The About the Spark Job Namespace section in the quick start guide was referencing quite heavily the old default value and how the empty string was enabling the deployment of spark applications on all namespaces. From the test I did with Helm chart v1.2.7 it seems to me that the empty list enables the execution of spark applications only in the spark-operator namespace. Based on this information I updated the doc describing this behavior, I may be wrong here, so please correct me if I am missing something

[vara-bonthu]

/lgtm

[google-oss-prow]

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