book: typo fixes (#143)

Author: TianyiShi2001

book/en-us/02-usability.md

@@ -203,7 +203,7 @@ E.g:
 int main() {
     std::vector<int> vec = {1, 2, 3, 4};
 
-    // after c++17, can be simplefied by using `auto`
+    // since c++17, can be simplified by using `auto`
     const std::vector<int>::iterator itr = std::find(vec.begin(), vec.end(), 2);
     if (itr != vec.end()) {
         *itr = 3;
@@ -214,7 +214,7 @@ int main() {
         *itr = 4;
     }
 
-    // should output: 1, 4, 3, 4. can be simplefied using `auto`
+    // should output: 1, 4, 3, 4. can be simplified using `auto`
     for (std::vector<int>::iterator element = vec.begin(); element != vec.end(); ++element)
         std::cout << *element << std::endl;
 }