maint: convert `...' to '...' in --help output

All affected lines end with \ or \n\, so run this command
until it produces no new changes (4 times):
  git grep -E -l '`[^ ]+'\''.*\\' src \
    |xargs perl -pi -e 's/`([^ ]+'\''.*\\)/'\''$1/'

1 files changed, 1 insertions, 1 deletions

diff --git a/src/md5sum.c b/src/md5sum.c
index 7024866a1..6f9c2c58d 100644
--- a/src/md5sum.c
+++ b/src/md5sum.c
@@ -204,7 +204,7 @@ The following three options are useful only when verifying checksums:\n\
 \n\
 The sums are computed as described in %s.  When checking, the input\n\
 should be a former output of this program.  The default mode is to print\n\
-a line with checksum, a character indicating input mode (`*' for binary,\n\
+a line with checksum, a character indicating input mode ('*' for binary,\n\
 space for text), and name for each FILE.\n"),
               DIGEST_REFERENCE);
       emit_ancillary_info ();