Remove incorrect "lambda" terminology

We only use "closures" to describe this construct.

Author: steveklabnik

src/fn/closures.md

@@ -1,8 +1,8 @@
 # Closures
 
-Closures in Rust, also called lambda expressions or lambdas, are functions that can capture 
-the enclosing environment. For example, a closure that captures the x 
-variable:
+Closures are functions that can capture the enclosing environment. For
+example, a closure that captures the x variable:
+
 ```Rust
 |val| val + x
 ```