book: typo fix (changkun#99)

Author: mrmiywj

book/en-us/02-usability.md

@@ -874,7 +874,7 @@ What we mainly mentioned above is a form of template parameters: type template p
 
 ```cpp
 template <typename T, typename U>
-    auto add(T t, U u) {
+auto add(T t, U u) {
     return t+u;
 }
 ```
@@ -1126,4 +1126,4 @@ This section introduces the enhancements to language usability in modern C++, wh
 
 ## Licenses
 
-<a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-nd/4.0/88x31.png" /></a><br />This work was written by [Ou Changkun](https://changkun.de) and licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License</a>. The code of this repository is open sourced under the [MIT license](../../LICENSE).
+<a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/"><img alt="Creative Commons License" style="border-width:0" src="https://i.creativecommons.org/l/by-nc-nd/4.0/88x31.png" /></a><br />This work was written by [Ou Changkun](https://changkun.de) and licensed under a <a rel="license" href="http://creativecommons.org/licenses/by-nc-nd/4.0/">Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License</a>. The code of this repository is open sourced under the [MIT license](../../LICENSE).