fix typo

R=rsc
https://golang.org/cl/152128

doc/go_tutorial.txt

@@ -426,7 +426,7 @@ declaration on line 31; it declares "r" and "e" to hold the two values,
 both of type "int" (although you'd have to look at the "syscall" package
 to see that).  Finally, line 35 returns two values: a pointer to the new "File"
 and the error.  If "syscall.Open" fails, the file descriptor "r" will
-be negative and "NewFile" will return "nil".
+be negative and "newFile" will return "nil".
 
 About those errors:  The "os" library includes a general notion of an error.
 It's a good idea to use its facility in your own interfaces, as we do here, for