springboot Fix typos in how to section of reference documentation

I think there are some typo in spring-boot-docs/src/docs/asciidoc/howto.adoc. Thank you for reading my PR.

Comment From: pivotal-issuemaster

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Comment From: pivotal-issuemaster

@YooInKeun Thank you for signing the Contributor License Agreement!

Comment From: snicoll

Well spotted @YooInKeun and thank you for making your first contribution to Spring Boot.

Comment From: YooInKeun

@snicoll Thank you!