[Github] Fix libc docs build (#70363)

llvm/llvm-project#69824 added libc build, but
missed the folder in ninja command, is causing failures.

ninja: fatal: chdir to 'docs-libc-html' - No such file or directory
ninja: Entering directory `docs-libc-html'

Author: jsji

.github/workflows/docs.yml

@@ -119,7 +119,7 @@ jobs:
         if: steps.docs-changed-subprojects.outputs.libc_any_changed == 'true'
         run: |
           cmake -B libc-build -GNinja -DCMAKE_BUILD_TYPE=Release -DLLVM_ENABLE_RUNTIMES="libc" -DLLVM_ENABLE_SPHINX=ON ./runtimes
-          TZ=UTC ninja -C docs-libc-html
+          TZ=UTC ninja -C libc-build docs-libc-html
       - name: Build LLD docs
         if: steps.docs-changed-subprojects.outputs.lld_any_changed == 'true'
         run: |