docs(style.css): update sidenav width to be in accordance with materi…

…al spec.

Refs 0318ca4
I believe 256px sidenav component width would be better accordance with material spec. incrementing keylines (64dp x 4 = 256dp) and this way sidenav component would fit perfectly to 8dp square baseline grid.
Spec detail: http://www.google.com/design/spec/layout/metrics-keylines.html#metrics-keylines-ratio-keylines

Closes #2392.

docs/app/css/style.css

@@ -186,9 +186,9 @@ code:not(.highlight) {
 .site-sidenav,
 .site-sidenav.md-locked-open-add-active,
 .site-sidenav.md-locked-open {
-  width: 254px;
-  min-width: 254px;
-  max-width: 254px;
+  width: 256px;
+  min-width: 256px;
+  max-width: 256px;
 }
 .site-sidenav > * {
   min-width: 218px;