fix(benchmarks-website/migrate): qualify Source::Local intra-doc link

`RUSTDOCFLAGS="-D warnings" cargo doc` fails the workspace docs build
with `unresolved link to Source::Local` on the module-level `//!`
comment in `migrate/src/source.rs`. Rustdoc can't resolve the
shorthand from the inner doc comment scope; qualify the link as
`crate::source::Source::Local` so the docs build is clean.

Verified locally with:

  RUSTDOCFLAGS="-D warnings" cargo doc --profile ci --no-deps \
    -p vortex-bench-server -p vortex-bench-migrate

passes cleanly.

Signed-off-by: Claude <noreply@anthropic.com>

Author: claude

benchmarks-website/migrate/src/source.rs

@@ -9,7 +9,7 @@
 //! path synchronous; the binary's hot path is single-threaded
 //! per-source already (DuckDB is a single-writer).
 //!
-//! For tests and offline runs, [`Source::Local`] accepts a local
+//! For tests and offline runs, [`Source::Local`](crate::source::Source::Local) accepts a local
 //! directory of dumps; the migrator's `--source` flag picks the
 //! variant.