docs/coverage.md: --csv option is outdated in syz-cover example

[Obstacle19]

When following syzkaller/docs/coverage.md, the example command for exporting function
coverage using syz-cover no longer works with the current implementation.

The documentation suggests using:
./bin/syz-cover --config --csv rawcover

However, the --csv flag is no longer defined. Running this command results in:
flag provided but not defined: -csv

According to the current syz-cover interface, coverage export formats are selected via
the -exports option. For example, raw function coverage can be exported using:
./bin/syz-cover --config -exports=rawcover

Updating the documentation accordingly would keep docs/coverage.md consistent with the
current behavior and avoid user confusion.

[a-nogikh]

Thanks!
I've sent #6639 to fix it.