TAJO-1061: Update build documentation

TAJO-1061: Update build documentation
I think there are some typos:
+ "If you doesn't" -> "If you don't"
+ "Thus, You must" -> "Thus, you must"

Beside, version "2.5.1" is not mandatory. It is just an example. Specifying a hadoop version is also not mandatory. Tajo can also work correctly at least in my system even though I do not specify a hadoop version. So, instead of saying "Thus, you must specify your hadoop version with maven build command as follows", I would like to suggest to write "Thus, we highly recommend that you specify your hadoop version with maven build command, for example, as follows"