Fix minimum rust version mismatch in lib.rs

[HoKim98]

This PR fixes a minimum rust version mismatch between /Cargo.toml and /src/lib.rs.

Expected Rust Version

ndarray/Cargo.toml

Line 6 in 8d91bc2

rust-version = "1.51"

Expected: 1.51.

Rust Version in docs.rs

ndarray/src/lib.rs

Line 74 in 8d91bc2

//! - **Requires Rust 1.49 or later**

Fixed: 1.49 -> 1.51

[adamreichold]

Thanks!