Rename 'designing a library' example (#911)

rename 'designing a library' example

library design in Rust is its own topic that this course does not cover; to avoid confusion, let's stick to describing things in terms of books

src/SUMMARY.md

@@ -64,7 +64,7 @@
   - [Lifetimes in Function Calls](ownership/lifetimes-function-calls.md)
   - [Lifetimes in Data Structures](ownership/lifetimes-data-structures.md)
 - [Exercises](exercises/day-1/afternoon.md)
-  - [Designing a Library](exercises/day-1/book-library.md)
+  - [Storing Books](exercises/day-1/book-library.md)
   - [Iterators and Ownership](exercises/day-1/iterators-and-ownership.md)
 
 

src/exercises/day-1/book-library.md

@@ -1,4 +1,4 @@
-# Designing a Library
+# Storing Books
 
 We will learn much more about structs and the `Vec<T>` type tomorrow. For now,
 you just need to know part of its API:
@@ -15,7 +15,7 @@ fn main() {
 }
 ```
 
-Use this to create a library application. Copy the code below to
+Use this to model a library's book collection. Copy the code below to
 <https://play.rust-lang.org/> and update the types to make it compile:
 
 ```rust,should_panic