content: change expression "under the covers" to "hood"

Fixes golang/tour#445

Change-Id: I69702bc5b3882188e8867e8112ec8e689e1ffb03
Reviewed-on: https://go-review.googlesource.com/123255
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>

content/methods.article

@@ -164,7 +164,7 @@ implementation, which could then appear in any package without prearrangement.
 
 * Interface values
 
-Under the covers, interface values can be thought of as a tuple of a value and a
+Under the hood, interface values can be thought of as a tuple of a value and a
 concrete type:
 
 	(value, type)