diff --git a/docs/interface.md b/docs/interface.md
index 4403922..cf2943f 100644
--- a/docs/interface.md
+++ b/docs/interface.md
@@ -655,7 +655,7 @@ type Foo = {
 ```typescript
 class Calculator implements Foo {
   result = 0;
-  add(num：number) {
+  add(num:number) {
     this.result += num;
     return this;
   }