Update while_let.md

Added `fn main()` as rustdoc didn't catch missing main function.

src/flow_control/while_let.md

@@ -31,24 +31,26 @@ loop {
 Using `while let` makes this sequence much nicer:
 
 ```rust,editable
-// Make `optional` of type `Option<i32>`
-let mut optional = Some(0);
-
-// This reads: "while `let` destructures `optional` into
-// `Some(i)`, evaluate the block (`{}`). Else `break`.
-while let Some(i) = optional {
-    if i > 9 {
-        println!("Greater than 9, quit!");
-        optional = None;
-    } else {
-        println!("`i` is `{:?}`. Try again.", i);
-        optional = Some(i + 1);
+fn main() {
+    // Make `optional` of type `Option<i32>`
+    let mut optional = Some(0);
+    
+    // This reads: "while `let` destructures `optional` into
+    // `Some(i)`, evaluate the block (`{}`). Else `break`.
+    while let Some(i) = optional {
+        if i > 9 {
+            println!("Greater than 9, quit!");
+            optional = None;
+        } else {
+            println!("`i` is `{:?}`. Try again.", i);
+            optional = Some(i + 1);
+        }
+        // ^ Less rightward drift and doesn't require
+        // explicitly handling the failing case.
     }
-    // ^ Less rightward drift and doesn't require
-    // explicitly handling the failing case.
+    // ^ `if let` had additional optional `else`/`else if`
+    // clauses. `while let` does not have these.
 }
-// ^ `if let` had additional optional `else`/`else if`
-// clauses. `while let` does not have these.
 ```
 
 ### See also: