Fix some more bugs in the tutorial

Tutorial code going out of date is going to be a recurring problem...

doc/tutorial/intro.md

@@ -27,10 +27,10 @@ a language can be made easier if the notation looks familiar. Rust is
 a curly-brace language in the tradition of C, C++, and JavaScript.
 
     fn fac(n: int) -> int {
-        let result = 1;
-        while n > 0 {
-            result *= n;
-            n -= 1;
+        let result = 1, i = 1;
+        while i <= n {
+            result *= i;
+            i += 1;
         }
         ret result;
     }

doc/tutorial/syntax.md

@@ -329,7 +329,7 @@ The compiler defines a few built-in syntax extensions. The most useful
 one is `#fmt`, a printf-style text formatting macro that is expanded
 at compile time.
 
-    std::io::writeln(#fmt("%s is %d", "the answer", 42));
+    std::io::println(#fmt("%s is %d", "the answer", 42));
 
 `#fmt` supports most of the directives that [printf][pf] supports, but
 will give you a compile-time error when the types of the directives
@@ -341,4 +341,4 @@ All syntax extensions look like `#word`. Another built-in one is
 `#env`, which will look up its argument as an environment variable at
 compile-time.
 
-    std::io::writeln(#env("PATH"));
+    std::io::println(#env("PATH"));