howto-index.sh: use the $( ... ) construct for command substitution

The Git CodingGuidelines prefer the $(...) construct for command
substitution instead of using the backquotes `...`.

The backquoted form is the traditional method for command
substitution, and is supported by POSIX.  However, all but the
simplest uses become complicated quickly.  In particular, embedded
command substitutions and/or the use of double quotes require
careful escaping with the backslash character.

The patch was generated by:

for _f in $(find . -name "*.sh")
do
   sed -i 's@`\(.*\)`@$(\1)@g' ${_f}
done

and then carefully proof-read.

Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto-index.sh

@@ -11,8 +11,8 @@ EOF
 
 for txt
 do
-	title=`expr "$txt" : '.*/\(.*\)\.txt$'`
-	from=`sed -ne '
+	title=$(expr "$txt" : '.*/\(.*\)\.txt$')
+	from=$(sed -ne '
 	/^$/q
 	/^From:[ 	]/{
 		s///
@@ -21,9 +21,9 @@ do
 		s/^/by /
 		p
 	}
-	' "$txt"`
+	' "$txt")
 
-	abstract=`sed -ne '
+	abstract=$(sed -ne '
 	/^Abstract:[ 	]/{
 		s/^[^ 	]*//
 		x
@@ -39,11 +39,11 @@ do
 		x
 		p
 		q
-	}' "$txt"`
+	}' "$txt")
 
 	if grep 'Content-type: text/asciidoc' >/dev/null $txt
 	then
-		file=`expr "$txt" : '\(.*\)\.txt$'`.html
+		file=$(expr "$txt" : '\(.*\)\.txt$').html
 	else
 		file="$txt"
 	fi