Fix typo in box.md (#2362)

Author: xrdelectro

src/smart-pointers/box.md

@@ -68,8 +68,8 @@ fn main() {
 - `Box` is like `std::unique_ptr` in C++, except that it's guaranteed to be not
   null.
 - A `Box` can be useful when you:
-  - have a type whose size that can't be known at compile time, but the Rust
-    compiler wants to know an exact size.
+  - have a type whose size can't be known at compile time, but the Rust compiler
+    wants to know an exact size.
   - want to transfer ownership of a large amount of data. To avoid copying large
     amounts of data on the stack, instead store the data on the heap in a `Box`
     so only the pointer is moved.