Darken @gray-light to meet accessibility guidelines

`@gray-light` darkens from #999 to #777, which puts `.text-muted` at the
threshold for the 4.5:1 WCAG minimum contrast[1]. #777:#fff is 4.48:1.

The “Example” headers in docs become #959595, which is contrast ratio
3:1, the minimum for larger text. Since the headers are less important
than the surrounding text, 3:1 is fine and an improvement on the
previous #bbb:#fff (1.92:1).

Fixes issue #13847.

[1] http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html

less/variables.less

@@ -10,7 +10,7 @@
 @gray-darker:            lighten(#000, 13.5%); // #222
 @gray-dark:              lighten(#000, 20%);   // #333
 @gray:                   lighten(#000, 33.5%); // #555
-@gray-light:             lighten(#000, 60%);   // #999
+@gray-light:             lighten(#000, 46.7%); // #777
 @gray-lighter:           lighten(#000, 93.5%); // #eee
 
 @brand-primary:         #428bca;