springboot Fix capitalization of Testcontainers in Howto docs

Unfortunately 3e3ff26 included a minor capitalization issue. TestContainers is incorrect; Testcontainers is the correct way to write the name.

Refs #20734

Comment From: pivotal-issuemaster

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Comment From: rnorth

@pivotal-issuemaster This is an Obvious Fix per https://cla.pivotal.io/about#obvious-fixes

Comment From: pivotal-issuemaster

@rnorth This Pull Request contains an obvious fix. Signing the Contributor License Agreement is not necessary.

Comment From: snicoll

@rnorth good catch, thank you.