Fix common mixup when instlling gems from the installation guide

doc/install/installation.md

@@ -238,10 +238,10 @@ Make sure to update username/password in config/database.yml.
     sudo gem install charlock_holmes --version '0.6.9'
-    # For mysql db
+    # For MySQL (note, the option says "without")
     sudo -u gitlab -H bundle install --deployment --without development test postgres
-    # Or For postgres db
+    # Or for PostgreSQL
     sudo -u gitlab -H bundle install --deployment --without development test mysql
 ## Configure Git