Fix typo in probe definition

[cwar]

Description

The documentation for the "Default Yet Configurable" startUpProbe contains a small typo where the u in startUp is not capitalized. This can cause some confusion when trying to incorporate this example config into a Druid configuration.

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This PR has:

• been tested on a real K8S cluster to ensure creation of a brand new Druid cluster works.
• been tested for backward compatibility on a real K*S cluster by applying the changes introduced here on an existing Druid cluster. If there are any backward incompatible changes then they have been noted in the PR description.
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added documentation for new or modified features or behaviors.

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Key changed/added files in this PR

• docs/features.md