Fixes errant parenthesis

Author: yawpitch

src/flow_control/if_let.md

@@ -104,7 +104,7 @@ Another benefit is that `if let` allows us to match non-parameterized enum varia
 Would you like a challenge? Fix the following example to use `if let`:
 
 ```rust,editable,ignore,mdbook-runnable
-// This enum purposely neither implements nor derives PartialEq).
+// This enum purposely neither implements nor derives PartialEq.
 // That is why comparing Foo::Bar == a fails below.
 enum Foo {Bar}