Found a little bug in the code of ch7-happycoin-as

There is a bug on this line 
`wasi.start(happycoinModule);` 
When I run this code, the wasi complained there is no '__start' function in the exports object.
And I checked the intermediate .js file, actually it only exported the 'memory' and 'getHappycoins'
So I changed it to:
`wasi.initialize(happycoinModule)`
which resolved this problem.
