doc: fix misspelt CSS properties.

R=golang-dev, rsc
CC=golang-dev
https://golang.org/cl/5498071

doc/all.css

@@ -81,15 +81,15 @@ h1#title {
   -webkit-box-shadow: 0 1px 1px #ccc;
 }
 #nav-main li:first-child {
-  -moz-border-top-left-radius: 4px;
+  -moz-border-radius-topleft: 4px;
   border-top-left-radius: 4px;
-  -moz-border-bottom-left-radius: 4px;
+  -moz-border-radius-bottomleft: 4px;
   border-bottom-left-radius: 4px;
 }
 #nav-main li:last-child {
-  -moz-border-top-right-radius: 4px;
+  -moz-border-radius-topright: 4px;
   border-top-right-radius: 4px;
-  -moz-border-bottom-right-radius: 4px;
+  -moz-border-radius-bottomright: 4px;
   border-bottom-right-radius: 4px;
 }
 #nav-main .quickref {