doc: fixing typos

concurrency.md

@@ -1,4 +1,4 @@
-% Concurrency and Paralellism
+% Concurrency and Parallelism
 
 Rust as a language doesn't *really* have an opinion on how to do concurrency or
 parallelism. The standard library exposes OS threads and blocking sys-calls

destructors.md

@@ -53,7 +53,7 @@ impl<T> Drop for Box<T> {
 
 and this works fine because when Rust goes to drop the `ptr` field it just sees
 a [Unique] that has no actual `Drop` implementation. Similarly nothing can
-use-after-free the `ptr` because when drop exits, it becomes inacessible.
+use-after-free the `ptr` because when drop exits, it becomes inaccessible.
 
 However this wouldn't work:
 

lifetimes.md

@@ -195,7 +195,7 @@ println!("{}", x);
 
 The problem here is is bit more subtle and interesting. We want Rust to
 reject this program for the following reason: We have a live shared reference `x`
-to a descendent of `data` when we try to take a mutable reference to `data`
+to a descendant of `data` when we try to take a mutable reference to `data`
 to `push`. This would create an aliased mutable reference, which would
 violate the *second* rule of references.