Updating schema validation for imagePullSecrets as expected by chart

[nitaliya]

Updated chart validation schema for imagePullSecrets to sync with the expectation of the chart
Previously expected validation by the schema is as mentioned below but the chart expects string instead of object type:

imagePullSecrets:
  - name: myregistry

Expected by the chart:

imagePullSecrets:
  - myregistry

[dmitryax]

Thanks for fixing this!