diff: short help: Correct -S description

`-S` shows changes that modify the number of occurrences of the
specified string, rather than only those that either completely remove
it or add it for the first time.

Signed-off-by: Junio C Hamano <[email protected]>

Author: ilya-bobyr

diff.h

@@ -606,7 +606,7 @@ void diffcore_fix_diff_index(void);
 "                try unchanged files as candidate for copy detection.\n" \
 "  -l<n>         limit rename attempts up to <n> paths.\n" \
 "  -O<file>      reorder diffs according to the <file>.\n" \
-"  -S<string>    find filepair whose only one side contains the string.\n" \
+"  -S<string>    find filepair who differ in the number of occurrences of string.\n" \
 "  --pickaxe-all\n" \
 "                show all files diff when -S is used and hit is found.\n" \
 "  -a  --text    treat all files as text.\n"