Rollup merge of rust-lang#32634 - varunvats:docs-fix, r=steveklabnik

Minor doc fixes in "Crates and Modules" and "Lifetimes" chapters

These commits fix a couple of (minor) issues in the _Crates and Modules_ and the _Lifetimes_ chapters of the book.

r? @steveklabnik

src/doc/book/crates-and-modules.md

@@ -118,7 +118,7 @@ build  deps  examples  libphrases-a7448e02a0468eaa.rlib  native
 `libphrases-hash.rlib` is the compiled crate. Before we see how to use this
 crate from another crate, let’s break it up into multiple files.
 
-# Multiple file crates
+# Multiple File Crates
 
 If each crate were just one file, these files would get very large. It’s often
 easier to split up crates into multiple files, and Rust supports this in two
@@ -190,13 +190,19 @@ mod farewells;
 ```
 
 Again, these declarations tell Rust to look for either
-`src/english/greetings.rs` and `src/japanese/greetings.rs` or
-`src/english/farewells/mod.rs` and `src/japanese/farewells/mod.rs`. Because
-these sub-modules don’t have their own sub-modules, we’ve chosen to make them
-`src/english/greetings.rs` and `src/japanese/farewells.rs`. Whew!
-
-The contents of `src/english/greetings.rs` and `src/japanese/farewells.rs` are
-both empty at the moment. Let’s add some functions.
+`src/english/greetings.rs`, `src/english/farewells.rs`,
+`src/japanese/greetings.rs` and `src/japanese/farewells.rs` or
+`src/english/greetings/mod.rs`, `src/english/farewells/mod.rs`,
+`src/japanese/greetings/mod.rs` and
+`src/japanese/farewells/mod.rs`. Because these sub-modules don’t have
+their own sub-modules, we’ve chosen to make them
+`src/english/greetings.rs`, `src/english/farewells.rs`,
+`src/japanese/greetings.rs` and `src/japanese/farewells.rs`. Whew!
+
+The contents of `src/english/greetings.rs`,
+`src/english/farewells.rs`, `src/japanese/greetings.rs` and
+`src/japanese/farewells.rs` are all empty at the moment. Let’s add
+some functions.
 
 Put this in `src/english/greetings.rs`:
 

src/doc/book/lifetimes.md

@@ -56,8 +56,8 @@ To fix this, we have to make sure that step four never happens after step
 three. The ownership system in Rust does this through a concept called
 lifetimes, which describe the scope that a reference is valid for.
 
-When we have a function that takes a reference by argument, we can be implicit
-or explicit about the lifetime of the reference:
+When we have a function that takes an argument by reference, we can be
+implicit or explicit about the lifetime of the reference:
 
 ```rust
 // implicit