Fix download links of Nightly Windows installers

The download links of Windows installers on the Nightly Rust page are using beta builds instead of nightly builds, which caused some confusions when I was setting up my env. Probably it's better to use the links of nightly builds here.

Author: Z1

src/doc/trpl/nightly-rust.md

@@ -26,8 +26,8 @@ $ sh rustup.sh --channel=nightly
 If you're on Windows, please download either the [32-bit installer][win32] or
 the [64-bit installer][win64] and run it.
 
-[win32]: https://static.rust-lang.org/dist/rust-1.0.0-beta-i686-pc-windows-gnu.msi
-[win64]: https://static.rust-lang.org/dist/rust-1.0.0-beta-x86_64-pc-windows-gnu.msi
+[win32]: https://static.rust-lang.org/dist/rust-nightly-i686-pc-windows-gnu.msi
+[win64]: https://static.rust-lang.org/dist/rust-nightly-x86_64-pc-windows-gnu.msi
 
 ## Uninstalling