spec: correct typo in string conversion examples.

R=golang-dev, dave, r
CC=golang-dev, remy
https://golang.org/cl/6378046

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of June 27, 2012",
+	"Subtitle": "Version of July 11, 2012",
 	"Path": "/ref/spec"
 }-->
 
@@ -3516,7 +3516,7 @@ the range of valid Unicode code points are converted to <code>"\uFFFD"</code>.
 
 <pre>
 string('a')       // "a"
-string(-1)        // "\ufffd" == "\xef\xbf\xbd "
+string(-1)        // "\ufffd" == "\xef\xbf\xbd"
 string(0xf8)      // "\u00f8" == "ø" == "\xc3\xb8"
 type MyString string
 MyString(0x65e5)  // "\u65e5" == "日" == "\xe6\x97\xa5"