fix: add test for @let without block, fix spans for error

maud/tests/warnings/let-without-block.rs

@@ -0,0 +1,9 @@
+use maud::html;
+
+fn main() {
+    html! {
+        p.@let x = 1; {
+            (x)
+        }
+    };
+}

maud/tests/warnings/let-without-block.stderr

@@ -0,0 +1,5 @@
+error: `@let` bindings are only allowed inside blocks
+ --> tests/warnings/let-without-block.rs:5:11
+  |
+5 |         p.@let x = 1; {
+  |           ^^^^^^^^^^^

maud_macros/src/ast.rs

@@ -104,12 +104,13 @@ impl DiagnosticParse for Markup {
         let markup = Self::diagnostic_parse_in_block(input, diagnostics)?;
 
         if let Self::ControlFlow(ControlFlow {
-            kind: ControlFlowKind::Let(expr),
+            kind: ControlFlowKind::Let(_),
             ..
         }) = &markup
         {
             diagnostics.push(
-                expr.span()
+                markup
+                    .span()
                     .error("`@let` bindings are only allowed inside blocks"),
             )
         }