Change aspnet:3.1 to aspnet:5.0 in first code example

[lambdakris]

The first code example was still referencing aspnet:3.1. This PR changes it to aspnet:5.0 to match the sdk version used as well as the runtime version used in later code examples.

Proposed changes

Change the runtime image layer in the first code example from 3.1 to 5.0 in order to match with the rest of the page.

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