doc: fix typo in Effective Go

R=adg, bradfitz
CC=golang-codereviews
https://golang.org/cl/44430043

doc/effective_go.html

@@ -350,7 +350,7 @@ not <code>encoding_base64</code> and not <code>encodingBase64</code>.
 </p>
 
 <p>
-The importer of a package will use the name to refer to its contents.
+The importer of a package will use the name to refer to its contents,
 so exported names in the package can use that fact
 to avoid stutter.
 (Don't use the <code>import .</code> notation, which can simplify