Fix Azure EventHubs source image reference

[tzununbekov]

Missed in https://github.com/triggermesh/charts/pull/180/files#diff-30d71f72ba5ddef0fe0438757bde96fcfb222ee49ce61e12651eb4c632048756

Resolves #183

I'm not familiar with the charts lifecycle, but we may need to re-create the release artifacts after this.