[book] [validation] fixed minor typos
minor #3733 [book] [validation] fixed minor typos (javiereguiluz)

This PR was merged into the 2.3 branch.

Discussion
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[book] [validation] fixed minor typos

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | 2.3+
| Fixed tickets | -

Commits
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693ba24 [book] [validation] fixed minor typos