Update _index.md

Author: duvallj

content/slides/_index.md

@@ -15,7 +15,7 @@ sort_by = "date"
 * [09_unsafe](09_unsafe.pdf)
 * [10_ffi](10_ffi.pdf)
 * [11_concurrency_1_threads](11_concurrency_1_threads.pdf)
-* [12_concurrency_1_async](12_concurrency_1_async.pdf)
+* [12_concurrency_2_async](12_concurrency_2_async.pdf)
 
 Previous semester's slides:
 * [F22](./F22/)