Fix clang Visual Studio build instructions

Change cd ..\.. to cd llvm-project (the former is probably a leftover
of the old svn instructions)

Committer: Adrian McCarthy <amccarth@google.com>

Differential Revision: https://reviews.llvm.org/D68321

clang/www/get_started.html

@@ -137,7 +137,7 @@ <h3 id="buildWindows">Using Visual Studio</h3>
   </li>
   <li>Run CMake to generate the Visual Studio solution and project files:
   <ul>
-    <li><tt>cd ..\..</tt>  (back to where you started)</li>
+    <li><tt>cd llvm-project</tt></li>
     <li><tt>mkdir build</tt> (for building without polluting the source dir)</li>
     <li><tt>cd build</tt></li>
     <li>