Fix some typos

Author: ykomatsu

src/doc/trpl/bibliography.md

@@ -48,7 +48,7 @@ Systems Level
 Language](http://www.cs.indiana.edu/~eholk/papers/hips2013.pdf). Early GPU work by Eric Holk.
 * [Parallel closures: a new twist on an old
   idea](https://www.usenix.org/conference/hotpar12/parallel-closures-new-twist-old-idea)
-  - not exactly about rust, but by nmatsakis
+  - not exactly about Rust, but by nmatsakis
 * [Patina: A Formalization of the Rust Programming
   Language](ftp://ftp.cs.washington.edu/tr/2015/03/UW-CSE-15-03-02.pdf). Early
   formalization of a subset of the type system, by Eric Reed.

src/doc/trpl/choosing-your-guarantees.md

@@ -204,7 +204,7 @@ borrow checker. Generally we know that such mutations won't happen in a nested f
 to check.
 
 For large, complicated programs, it becomes useful to put some things in `RefCell`s to make things
-simpler. For example, a lot of the maps in [the `ctxt` struct][ctxt] in the rust compiler internals
+simpler. For example, a lot of the maps in [the `ctxt` struct][ctxt] in the Rust compiler internals
 are inside this wrapper. These are only modified once (during creation, which is not right after
 initialization) or a couple of times in well-separated places. However, since this struct is
 pervasively used everywhere, juggling mutable and immutable pointers would be hard (perhaps

src/doc/trpl/concurrency.md

@@ -150,7 +150,7 @@ owners!
 So, we need some type that lets us have more than one reference to a value and
 that we can share between threads, that is it must implement `Sync`.
 
-We'll use `Arc<T>`, rust's standard atomic reference count type, which
+We'll use `Arc<T>`, Rust's standard atomic reference count type, which
 wraps a value up with some extra runtime bookkeeping which allows us to
 share the ownership of the value between multiple references at the same time.
 

src/doc/trpl/error-handling.md

@@ -279,7 +279,7 @@ fn extension(file_name: &str) -> Option<&str> {
 }
 ```
 
-One other pattern that we find is very common is assigning a default value to
+One other pattern that we find it very common is assigning a default value to
 the case when an `Option` value is `None`. For example, maybe your program
 assumes that the extension of a file is `rs` even if none is present. As you
 might imagine, the case analysis for this is not specific to file