Default one to rocksdb:x64-windows

Summary:
The default one will try to install rocksdb:x86-windows, which would lead to failing of the build at the last step (CMake Error, Rocksdb only supports x64). Because it will try to install a serials of x86 version package, and those cannot proceed to rocksdb:x86-windows building. By using rocksdb:x64-windows, we can make sure to install x64 version.
Tested on Win10 x64.
Closes facebook#2941

Differential Revision: D5937139

Pulled By: sagar0

fbshipit-source-id: 15637fe23df59326a0e607bd4d5c48733e20bae3

INSTALL.md

@@ -114,7 +114,7 @@ to build a portable binary, add `PORTABLE=1` before your make commands, like thi
   * For building with MS Visual Studio 13 you will need Update 4 installed.
   * Read and follow the instructions at CMakeLists.txt
   * Or install via [vcpkg](https://github.com/microsoft/vcpkg) 
-       * run `vcpkg install rocksdb`
+       * run `vcpkg install rocksdb:x64-windows`
 
 * **AIX 6.1**
     * Install AIX Toolbox rpms with gcc