[cxx-interop] Fix typo in getting started, and update example for creating the cxx module

[cabmeurer]

Fixed the typo in the cmake section and updated the creating module section with a more accurate example.

I found that when you create a C++ module, and defined file for header in the module doesnt contain the implimentation for the methods, an error is produced stating Undefined symbol: _cxxFunction, clang: error: linker command failed with exit code 1 (use -v to see invocation)

I updated the docs to have the implementation of methods in the header file, however we could change the example as seen below:

//In module.modulemap
module Cxx {
    header "Cxx.cpp"
}

// in Cxx.hpp
#ifndef Cxx_hpp
#define Cxx_hpp
int cxxFunction(int n);

#endif

//in Cxx.cpp
#include "Cxx.hpp"

int cxxFunction(int n) {
    return n;
}

[cabmeurer]

@hyp @zoecarver

[zoecarver]

@swift-ci please smoke test.