Fix usage for --datadir option #2122

Job	Run time
check (libclamav, (iana_cctld\|bytecode_api_\|bytecode_hooks\|rijndael\|yara\|inffixed\|inflate\|queue\|n...	19s
check (libfreshclam)	22s
check (clamav-milter)	19s
check (clambc)	20s
check (clamconf)	20s
check (clamd)	21s
check (clamdscan)	19s
check (clamdtop)	19s
check (clamonacc, (c-thread-pool\|fts\|priv_fts))	18s
check (clamscan)	21s
check (clamsubmit)	25s
check (freshclam)	19s
check (libfreshclam)	52s
check (common)	21s
check (sigtool)	22s
check (examples)	19s
check (win32/compat)	19s
	6m 15s

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