fixed a typo in the lifetime.md

src/scope/lifetime.md

@@ -1,6 +1,6 @@
 # Lifetimes
 
-A *lifetime* is a construct of the compiler (or more specifically, its *borrow
+A *lifetime* is a construct the compiler (or more specifically, its *borrow
 checker*) uses to ensure all borrows are valid. Specifically, a variable's
 lifetime begins when it is created and ends when it is destroyed. While
 lifetimes and scopes are often referred to together, they are not the same.