docs: Make some tiny modification about spelling

src/doc/book/primitive-types.md

@@ -7,7 +7,7 @@ of these ones, as well, but these are the most primitive.
 
 # Booleans
 
-Rust has a built in boolean type, named `bool`. It has two values, `true` and `false`:
+Rust has a built-in boolean type, named `bool`. It has two values, `true` and `false`:
 
 ```rust
 let x = true;
@@ -89,13 +89,13 @@ Unsigned types use a `u` for their category, and signed types use `i`. The `i`
 is for ‘integer’. So `u8` is an eight-bit unsigned number, and `i8` is an
 eight-bit signed number.
 
-## Fixed size types
+## Fixed-size types
 
-Fixed size types have a specific number of bits in their representation. Valid
+Fixed-size types have a specific number of bits in their representation. Valid
 bit sizes are `8`, `16`, `32`, and `64`. So, `u32` is an unsigned, 32-bit integer,
 and `i64` is a signed, 64-bit integer.
 
-## Variable sized types
+## Variable-size types
 
 Rust also provides types whose size depends on the size of a pointer of the
 underlying machine. These types have ‘size’ as the category, and come in signed