Update rand-range.md

[mingscraft]

fixes #631 incorrect usage of rng.gen_range

Things to check before submitting a PR

• [x] the tests are passing locally with cargo test
• [x] cookbook renders correctly in mdbook serve -o
• [x] commits are squashed into one and rebased to latest master
• [x] PR contains correct "fixes #ISSUE_ID" clause to autoclose the issue on PR merge
  • if issue does not exist consider creating it or remove the clause
• [x] spell check runs without errors ./ci/spellcheck.sh
• [x] link check runs without errors link-checker ./book
• [x] non rendered items are in sorted order (links, reference, identifiers, Cargo.toml)
• [x] links to docs.rs have wildcard version https://docs.rs/tar/*/tar/struct.Entry.html
• [x] example has standard error handling
• [x] code identifiers in description are in hyperlinked backticks

Things to do after submitting PR

• [ ] check if CI is happy with your PR

Thank you for reading, you may now delete this text! Thank you! :smile:

[djc]

This has been fixed in #627, and can be closed.