book: fix a minor typo (changkun#97)

fipped · web-flow · commit 1fcfd36964c9 · 2020-04-23T08:28:57.000+02:00
diff --git a/book/zh-cn/02-usability.md b/book/zh-cn/02-usability.md
@@ -779,7 +779,7 @@ int main() {
 
 ```cpp
 template <typename T, typename U>
-    auto add(T t, U u) {
+auto add(T t, U u) {
     return t+u;
 }
 ```

Original file line number	Diff line number	Diff line change
`@@ -779,7 +779,7 @@ int main() {`
`779`	`779`
`780`	`780`	```cpp
`781`	`781`	`template <typename T, typename U>`
`782`		`- auto add(T t, U u) {`
	`782`	`+auto add(T t, U u) {`
`783`	`783`	`return t+u;`
`784`	`784`	`}`
`785`	`785`	```