typos: fix a grabbag of typos all over the place

src/doc/grammar.md

@@ -773,7 +773,7 @@ bound := path | lifetime
 
 ## Type kinds
 
-**FIXME:** this this probably not relevant to the grammar...
+**FIXME:** this is probably not relevant to the grammar...
 
 # Memory and concurrency models
 

src/doc/trpl/concurrency.md

@@ -195,7 +195,7 @@ our value if it's immutable, but we want to be able to mutate it, so we need
 something else to persuade the borrow checker we know what we're doing.
 
 It looks like we need some type that allows us to safely mutate a shared value,
-for example a type that that can ensure only one thread at a time is able to
+for example a type that can ensure only one thread at a time is able to
 mutate the value inside it at any one time.
 
 For that, we can use the `Mutex<T>` type!

src/doc/trpl/iterators.md

@@ -302,7 +302,7 @@ This will print
 
 `filter()` is an adapter that takes a closure as an argument. This closure
 returns `true` or `false`. The new iterator `filter()` produces
-only the elements that that closure returns `true` for:
+only the elements that the closure returns `true` for:
 
 ```rust
 for i in (1..100).filter(|&x| x % 2 == 0) {