jump plugin: fix autocompletion with single mark

Autocompletion fails if there's only one mark, since the ls command
will not display the parent directory with the trailing colon.
Handling the single mark case separately and validating the symlink
explicitly, resolves the issue.

plugins/jump/jump.plugin.zsh

@@ -38,7 +38,13 @@ marks() {
 }
 
 _completemarks() {
+	if [[ $(ls "${MARKPATH}" | wc -l) -gt 1 ]]; then
 		reply=($(ls $MARKPATH/**/*(-) | grep : | sed -E 's/(.*)\/([_\da-zA-Z\-]*):$/\2/g'))
+	else
+		if readlink -e "${MARKPATH}"/* &>/dev/null; then
+			reply=($(ls "${MARKPATH}"))
+		fi
+	fi
 }
 compctl -K _completemarks jump
 compctl -K _completemarks unmark