zsh autocomplete: replace use of sed with cut

Signed-off-by: Andre Eriksson <aepubemail@gmail.com>

contrib/completion/zsh/_docker-compose

@@ -53,7 +53,7 @@ __docker-compose_services_with_key() {
         | sed -n 's/^  //p' \
         | awk '/^[a-zA-Z0-9]/{printf "\n"};{printf $0;next;}' \
         | grep " \+$1:" \
-        | sed "s/:.*//g" \
+        | cut -d: -f1 \
         | grep -Ev "^(${already_selected})$"
 }