Fix typo in IntersectionObserver test observer-exceptions.html

This fixes a typo in the description of a test case.

TBR=szager@chromium.org

Change-Id: I485f41c68bfa747c794ef9add2b96b296cfdf73e
Reviewed-on: https://chromium-review.googlesource.com/1173151Reviewed-by: Ali Juma <ajuma@chromium.org>
Commit-Queue: Ali Juma <ajuma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582686}

third_party/WebKit/LayoutTests/external/wpt/intersection-observer/observer-exceptions.html

@@ -19,7 +19,7 @@ test(function () {
   assert_throws("SYNTAX_ERR", function() {
     new IntersectionObserver(e => {}, {rootMargin: "1"})
   })
-}, 'IntersectionObserver constructor witth { rootMargin: "1" }');
+}, 'IntersectionObserver constructor with { rootMargin: "1" }');
 
 test(function () {
   assert_throws("SYNTAX_ERR", function() {