fixed a missing description of the MSRV option for manual_repeat_n (rust-lang#14081)

I added `manual_repeat_n` lint before, which has the `MSRV` option.
However, I forgot to add this lint to the lint configuration.

changelog: none

Author: y21

book/src/lint_configuration.md

@@ -750,6 +750,7 @@ The minimum rust version that the project supports. Defaults to the `rust-versio
 * [`manual_pattern_char_comparison`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_pattern_char_comparison)
 * [`manual_range_contains`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_range_contains)
 * [`manual_rem_euclid`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_rem_euclid)
+* [`manual_repeat_n`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_repeat_n)
 * [`manual_retain`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_retain)
 * [`manual_split_once`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_split_once)
 * [`manual_str_repeat`](https://rust-lang.github.io/rust-clippy/master/index.html#manual_str_repeat)

clippy_config/src/conf.rs

@@ -619,6 +619,7 @@ define_Conf! {
         manual_pattern_char_comparison,
         manual_range_contains,
         manual_rem_euclid,
+        manual_repeat_n,
         manual_retain,
         manual_split_once,
         manual_str_repeat,