Added newlines to warning message

[juansc]

Addresses https://github.com/lsd-rs/lsd/issues/868. After building

> ./target/release/lsd
Warning: the 'themes' directory is deprecated, use 'colors.yaml' instead.

 build.rs     Cargo.toml     ci           doc       README.md      src      tests
 Cargo.lock   CHANGELOG.md   CODEOWNERS   LICENSE   rustfmt.toml   target

---

TODO

• [x] Use cargo fmt
• [x] Add necessary tests (Not needed)
• [x] Add changelog entry (Not a significant change)
• [x] Update default config/theme in README (if applicable)
• [x] Update man page at lsd/doc/lsd.md (if applicable)

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