fix update llvm build command

Author: gnana-ganesh-tw

src/backend/updating-llvm.md

@@ -90,7 +90,7 @@ so let's go through each in detail.
    `src/llvm-project` to ensure submodule updates aren't reverted.
    Some commands you should execute are:
 
-   * `./x build src/llvm` - test that LLVM still builds
+   * `./x build src/llvm-project` - test that LLVM still builds
    * `./x build src/tools/lld` - same for LLD
    * `./x build` - build the rest of rustc