scripts/process-markdown: Fix usage of readlink

Had used 'readlink -f' to canonicalize input and output paths.
However, this fails when the output directory does not exist yet.

Now uses 'readlink -m' instead to avoid this problem.

src/scripts/process-markdown

@@ -12,13 +12,13 @@
 # output markdown.
 
 set -e
- if [ $# != 2 ]; then
+if [ $# != 2 ]; then
     echo "Usage: $0 <input> <output>" >&2
     exit 1
 fi
 
-input=$(readlink -f $1)
-output=$(readlink -f $2)
+input=$(readlink -m $1)
+output=$(readlink -m $2)
 
 [ -d $(dirname "$output") ] || mkdir -p "$(dirname $output)"
 cd $(dirname $output)