Fix typo.

Typo reported as Polymer/old-docs-site#1033.

core-list.html

@@ -25,7 +25,7 @@
 explicit CSS `height` property set via a class or inline style, or else is sized
 by other layout means (e.g. `flex` or `fit`).  Alternatively, `core-list` can
 delegate scrolling to a scrollable element that contains the list by setting the
-`scrollTarget` property, and the same explicit sizing requiremets will apply
+`scrollTarget` property, and the same explicit sizing requirements will apply
 to that element.
 
 ### Template model