fix:修正一处代码错误。

source/_posts/rust-macro.md

@@ -234,7 +234,8 @@ mod client;
 
 ```rust
 macro_rules! calculate {
-  (eval $e:expr store to $a:ident) => {{
+  // 注意，expr类型描述后面可以接的内容限制
+  (eval $e:expr, store to $a:ident) => {{
     let $a = $e;
   }}
 }
@@ -246,7 +247,7 @@ macro_rules! calculate {
 fn main() {
   let variable: usize = 0;
   calculate! {
-    eval (100 + 50) / (29 * 16) store to variable
+    eval (100 + 50) / (29 * 16), store to variable
   }
 }
 ```