Update document CONTRIBUTING.md section Test

[XuJiandong]

In the section Test, The correct way to run bsc is as following:

./darwin/bsc.exe -I stdlib-406 myTestFile.ml

not runtime folder.

[bobzhang]

hi @XuJiandong, it should be ./darwin/bsc.exe myTestFile.ml, would you send a PR? thanks

[bobzhang]

fixed in master