doc: fix typo in Effective Go: s/ReaderWriter/ReadWriter/

Change-Id: I3bfe1b11265f0def4701faf2cfc1ad10a666a473
Reviewed-on: https://go-review.googlesource.com/79596
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/effective_go.html

@@ -2790,7 +2790,7 @@ <h2 id="embedding">Embedding</h2>
 <p>
 If we need to refer to an embedded field directly, the type name of the field,
 ignoring the package qualifier, serves as a field name, as it did
-in the <code>Read</code> method of our <code>ReaderWriter</code> struct.
+in the <code>Read</code> method of our <code>ReadWriter</code> struct.
 Here, if we needed to access the
 <code>*log.Logger</code> of a <code>Job</code> variable <code>job</code>,
 we would write <code>job.Logger</code>,