SpringBoot Fix typo in Spring Session documentation

Fix typo in Spring Session documentation (obvious fix)

Comment From: pivotal-issuemaster

@PascalSchumacher Please sign the Contributor License Agreement!

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Comment From: PascalSchumacher

@pivotal-issuemaster This is an Obvious Fix

Comment From: pivotal-issuemaster

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

Comment From: wilkinsona

Thanks very much, @PascalSchumacher.