Typos:
* "**The** are two important models" => "**There** are two important models"
*"A web server provides information in response to the **query** sent by its clients." => "A web server provides information in response to the **queries** sent by its clients."
"The `SP` terminal corresponds to the white space characters." => ""The `SP` terminal corresponds to the white space character."
* "A `username` contains at least one letter and up to 8 letters. **User names** are case-sensitive as.." => "A `username` contains at least one letter and up to 8 letters. **Usernames** are case-sensitive as.."