Use the correct order of arguments for warnings.warn function

[alwx]

There is a mistake with the order of arguments in warnings.warn function call, and because of this mistake the library errors when CFFI C extension is not available. It basically means the pure Python implementation of CFFI cannot be used.

Here is the error:

c:\users\runneradmin\appdata\local\pypoetry\cache\virtualenvs\rasa-8anjnmh4-py3.8\lib\site-packages\crc32c\__init__.py:29: in <module>
51
    warnings.warn(RuntimeWarning, "_SLOW_CRC32C_WARNING",)
52
E   TypeError: category must be a Warning subclass, not 'str'

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[alwx]

@googlebot I signed it!

[tseaver]

@alwx Thanks for the patch! I noticed the same error yesterday and folded pretty much the same change into #30.