fixed wrong inner links

Author: rotarymars

lang/cpp26/nice_placeholder_with_no_name.md

@@ -25,7 +25,7 @@ int main() {
 ```
 
 ## <a id="relative-page" href="#relative-page">関連項目</a>
-- [\[\[maybe_unused\]\]属性 \[P0212R1\]](/lang/cpp17/maybe_unused.html)
+- [\[\[maybe_unused\]\]属性 \[P0212R1\]](/lang/cpp17/maybe_unused.md)
 
 ## 参照
 - [P2169R4 A nice placeholder with no name](https://open-std.org/jtc1/sc22/wg21/docs/papers/2023/p2169r4.pdf)