[www] Fix Ninja build instructions on Windows

The `clang` target used in the line below is only generated with `LLVM_ENABLE_PROJECTS=clang`.

Without this change, running `ninja clang` will fail with:
```
ninja: error: unknown target 'clang', did you mean 'clean'?
```

Reviewed By: xgupta

Differential Revision: https://reviews.llvm.org/D112257