fix typo ccp4in -> ccp4gz

I also got this:
```
NDKmol/CCP4Reader.cpp: In constructor ‘CCP4file::CCP4file(std::string)’:
NDKmol/CCP4Reader.cpp:45:45: error: cannot convert ‘FILE* {aka _IO_FILE*}’ to ‘gzFile’ for argument ‘1’ to ‘off_t gzseek(gzFile, off_t, int)’
    gzseek(ccp4in, 256 * 4 + NSYMBT, SEEK_SET);

```
I guess because of typo