[www] Fix Ninja build instructions on Windows

[triplef]

Summary

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'?

clang/www/get_started.html

<li>Open a developer command prompt with the appropriate environment.
     </ul>
   </li>
   <li><tt>mkdir build_ninja</tt> (or <tt>build</tt>, or use your own
     organization)</li>
   <li><tt>cd build_ninja</tt></li>
   <li><tt>set CC=cl</tt> (necessary to force CMake to choose MSVC over mingw GCC
     if you have it installed)</li>
   <li><tt>set CXX=cl</tt></li>
-  <li><tt>cmake -GNinja ..\llvm</tt></li>
+  <li><tt>cmake -GNinja -DLLVM_ENABLE_PROJECTS=clang ..\llvm</tt></li>
   <li><tt>ninja clang</tt> This will build just clang.</li>
   <li><tt>ninja check-clang</tt> This will run the clang tests.</li>
 </ol>
 
 <h2 id="driver">Clang Compiler Driver (Drop-in Substitute for GCC)</h2>
 
 <p>The <tt>clang</tt> tool is the compiler driver and front-end, which is
 designed to be a drop-in replacement for the <tt>gcc</tt> command.  Here are