Fix typo word

[truebit]

fix issue #122

[aisk]

I think we should have an alias for ThriftGrammerError for don't break unit test and users' codes.

And maybe we can declare a module level __getattr__ which descibed in PEP 562, for users who are using Python3.7 or latter versions can got a deprecated warning.

[truebit]

IMHO, this change do not need to consider backward compatibility due to the ROI of this change. Anyone used old ThriftGrammerError would be easy to find out the changed class name.

[ethe]

I think it would be better to keep backward compatibility, I have to release a minor version after this pull request be merged, it is not necessary

[aisk]

The only cocern is the error message is changed, but I think this will only affect thriftpy2's test.