Book: Fix the link to burn-train in "Learner" page

[towerpark]

Pull Request Template

Checklist

• Confirmed that run-checks all script has been executed.
  I couldn't run through all the checks due to errors from compiling "burn-autodiff".
• Made sure the book is up to date with changes in this PR.

Related Issues/PRs

None.

Changes

• Problem:
  The "crates" directory part is missing in the link to the burn-train crate in the "Learner" page.
• Solution:
  Add the missing "crates/" to the link.

Testing

• cargo xtask books burn open
• Unwrapping the paragraph into one line and ensuring by diff that the only actual change is "crates/" being added.