PATCH: silence Shellcheck warning

This patch silences a warning from Shellcheck about using old-style
`...` command substitutions.

Ben


commit 4d35c7aae97234bf055519075ef03cd4090a1dfc
Author: Ben Elliston <[email protected]>
Date:   Sun Jul 22 08:22:44 2018 +1000

    * missing: Use $(..) command substitution syntax.

diff --git a/lib/missing b/lib/missing
index 625aeb1..10dc11e 100755
--- a/lib/missing
+++ b/lib/missing
@@ -1,7 +1,7 @@
 #! /bin/sh
 # Common wrapper for a few potentially missing GNU programs.
 
-scriptversion=2018-03-07.03; # UTC
+scriptversion=2018-07-21.22; # UTC
 
 # Copyright (C) 1996-2018 Free Software Foundation, Inc.
 # Originally written by Fran,cois Pinard <[email protected]>, 1996.
@@ -129,10 +129,10 @@ program_details ()
 give_advice ()
 {
   # Normalize program name to check for.
-  normalized_program=`echo "$1" | sed '
+  normalized_program=$(echo "$1" | sed '
     s/^gnu-//; t
     s/^gnu//; t
-    s/^g//; t'`
+    s/^g//; t')
 
   printf '%s\n' "'$1' is $msg."

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