Update docs/examples/sticky-footer.html

again, "it's" to "its" for grammar fun!

docs/examples/sticky-footer.html

@@ -40,7 +40,7 @@
         min-height: 100%;
         height: auto !important;
         height: 100%;
-        /* Negative indent footer by it's height */
+        /* Negative indent footer by its height */
         margin: 0 auto -60px;
       }