Update options.md (zyedidia#3615)

Author: antoine2711

runtime/help/options.md

@@ -450,11 +450,11 @@ Here are the available options:
 * `tabhighlight`: inverts the tab characters' (filename, save indicator, etc)
    colors with respect to the tab bar.
 
-    default value: false
+    default value: `false`
 
 * `tabreverse`: reverses the tab bar colors when active.
 
-    default value: true
+    default value: `true`
 
 * `tabsize`: the size in spaces that a tab character should be displayed with.