Fix dropdown CSS in EDU account addition flow

Bug: 1071608
Change-Id: Ic756c2d5b2a7536db82eddfc74c3f2a7b393787a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2153154Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Anastasiia N <anastasiian@chromium.org>
Cr-Commit-Position: refs/heads/master@{#760125}

chrome/browser/resources/chromeos/edu_login/edu_login_parent_signin.html

@@ -26,7 +26,7 @@
     --icon-size: 24px;
     flex-shrink: 0;
     height: var(--icon-size);
-    margin-block-start: auto;
+    margin-inline-start: auto;
     width: var(--icon-size);
   }