CI: Fix shell syntax in circle.yml

Forgot line continuation
diff --git a/circle.yml b/circle.yml
index 41f322d..75c3667 100644
--- a/circle.yml
+++ b/circle.yml
@@ -186,11 +186,11 @@
     # Download the latest mathquill artifacts.
     - |-
       prev_imgs="$(curl https://circleci.com/api/v1/project/mathquill/mathquill/latest/artifacts \
-                    | tee /dev/stderr
+                    | tee /dev/stderr \
                     | $json -a url pretty_path -d '\n\t' \
                     | grep '\.png$' \
                     | grep -v 'PREV_' \
-                    | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:"
+                    | sed "s:\$CIRCLE_ARTIFACTS/imgs/:-o $CIRCLE_ARTIFACTS/imgs/PREV_:" \
                     | tee /dev/stderr)"
       test -z "$prev_imgs" || curl $prev_imgs