[doc][installing] Updated `Installing the library` doc
[doc][installing] Updated `Installing the library` doc

  * Replaced `Swiftmailer` by `Swift Mailer`, as this is the most accepted form in the rest of the documentation
  * Removed user and host information from command line prompt (e.g. `$ ls` instead of `chrisbook:build chris$ ls`). This change improves readability and follows the principles of the Symfony documentation.
  * Fixed a few minor typos and wrapped some line to 80 character limit.

robertklein/swiftmailer,peterkokot/swiftmailer,UofSC/php-shared-functions,Dubz/swiftmailer,handautsav/swiftmailer,Pardot/swiftmailer,swiftmailer/swiftmailer,peterkokot/swiftmailer,papagel/swiftmailer,zghosts/swiftmailer,Pardot/swiftmailer,tweakers-dev/swiftmailer,Pardot/swiftmailer,UofSC/php-shared-functions,tweakers-dev/swiftmailer,voku/swiftmailer,Chhunlong/swiftmailer,zghosts/swiftmailer,gburyakovsky/swiftmailer,swiftmailer/swiftmailer,fvosberg/swiftmailer,gencer/swiftmailer,fvosberg/swiftmailer,jamesggordon/swiftmailer,jamesggordon/swiftmailer,voku/swiftmailer,Rotzbua/swiftmailer,gburyakovsky/swiftmailer,kuslahne/swiftmailer,Pardot/swiftmailer,UofSC/php-shared-functions,denpamusic/swiftmailer,c960657/swiftmailer,papagel/swiftmailer,Chhunlong/swiftmailer,michabbb-backup/swiftmailer,kuslahne/swiftmailer,deguif/swiftmailer,c960657/swiftmailer,Rotzbua/swiftmailer,gencer/swiftmailer,michabbb-backup/swiftmailer,Dubz/swiftmailer,handautsav/swiftmailer,denpamusic/swiftmailer,robertklein/swiftmailer,deguif/swiftmailer,peterkokot/swiftmailer