Fix typo in intro2.rs to correctly print "Hello world!" and update .rustlings-state.txt

.rustlings-state.txt

@@ -0,0 +1,6 @@
+DON'T EDIT THIS FILE!
+
+variables1
+
+intro1
+intro2

exercises/00_intro/intro2.rs

@@ -1,4 +1,4 @@
 fn main() {
     // TODO: Fix the code to print "Hello world!".
-    printline!("Hello world!");
-}
+    println!("Hello world!");
+}