Fix -S/-z usage() text

[rowlap]

fatcat/src/fatcat.cpp

Lines 50 to 51 in 98e1cd5

	cout << "* -S: write scamble data in unallocated sectors" << endl;
	cout << "* -z: write scamble data in unallocated sectors" << endl;

says
-S: write scamble data in unallocated sectors
-z: write scamble data in unallocated sectors

but should say this instead
-S: write scramble data in unallocated sectors
-z: write zero data in unallocated sectors