Typo (?) in help message

According to the help message ([fast_align.cc:307](https://github.com/clab/fast_align/blob/master/src/fast_align.cc#L307)) the option -c outputs a conditional probability table, but the actual option appears to be -p ([fast_align.cc:89](https://github.com/clab/fast_align/blob/master/src/fast_align.cc#L89)).