Rename unstable book correctly

Author: mark-i-m

src/doc/unstable-book/src/language-features/repri128.md renamed to src/doc/unstable-book/src/language-features/repr128.md

@@ -1,15 +1,15 @@
-# `i128`
+# `repri128`
 
 The tracking issue for this feature is: [#35118]
 
 [#35118]: https://github.com/rust-lang/rust/issues/35118
 
 ------------------------
 
-The `i128` feature adds support for `#[repr(u128)]` on `enum`s.
+The `repr128` feature adds support for `#[repr(u128)]` on `enum`s.
 
 ```rust
-#![feature(repri128)]
+#![feature(repr128)]
 
 #[repr(u128)]
 enum Foo {