doc/effective_go.html: use correct name in Interface Checks section

Fixes #4897.

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/7377061

doc/effective_go.html

@@ -3041,7 +3041,6 @@ Similarly, assigning <code>greeting</code> to a blank identifier
 will silence the unused variable error.
 </p>
 {{code "/doc/progs/unused2.go" `/package/` `$`}}
-<p>
 
 <h3 id="blank_import">Import for side effect</h3>
 
@@ -3103,7 +3102,7 @@ cause the compiler to verify this automatically.
 A declaration can be used to add such a check:
 </p>
 <pre>
-var _ json.Marshaler = (*MyMessage)(nil)
+var _ json.Marshaler = (*RawMessage)(nil)
 </pre>
 <p>
 As part of type-checking this static assignment of a