docs: replace x with 'x'

It is a string.

Author: kenjis

user_guide_src/source/installation/upgrade_4xx.rst

@@ -102,7 +102,7 @@ Libraries
 =========
 
 - Your app classes can still go inside **app/Libraries**, but they don't have to.
-- Instead of CI3's ``$this->load->library(x);`` you can now use
+- Instead of CI3's ``$this->load->library('x');`` you can now use
   ``$this->x = new X();``, following namespaced conventions for your component.
 
 Helpers
@@ -156,7 +156,7 @@ Upgrading Libraries
 *******************
 
 - Your app classes can still go inside **app/Libraries**, but they don't have to.
-- Instead of CI3's ``$this->load->library(x);`` you can now use ``$this->x = new X();``,
+- Instead of CI3's ``$this->load->library('x');`` you can now use ``$this->x = new X();``,
   following namespaced conventions for your component.
 - Some libraries from CodeIgniter 3 no longer exists in Version 4. For all these
   libraries, you have to find a new way to implement your functions. These

user_guide_src/source/installation/upgrade_models.rst

@@ -24,7 +24,7 @@ Upgrade Guide
 2. Add this line just after the opening php tag: ``namespace App\Models;``.
 3. Below the ``namespace App\Models;`` line add this line: ``use CodeIgniter\Model;``.
 4. Replace ``extends CI_Model`` with ``extends Model``.
-5. Instead of CI3's ``$this->load->model(x);``, you would now use ``$this->x = new X();``, following namespaced conventions for your component. Alternatively, you can use the :php:func:`model()` function: ``$this->x = model('X');``.
+5. Instead of CI3's ``$this->load->model('x');``, you would now use ``$this->x = new X();``, following namespaced conventions for your component. Alternatively, you can use the :php:func:`model()` function: ``$this->x = model('X');``.
 
 If you use sub-directories in your model structure you have to change the namespace according to that.
 Example: You have a version 3 model located in **application/models/users/user_contact.php** the namespace has to be ``namespace App\Models\Users;`` and the model path in the version 4 should look like this: **app/Models/Users/UserContact.php**

user_guide_src/source/installation/upgrade_views.rst

@@ -15,7 +15,7 @@ What has been changed
 =====================
 
 - Your views look much like before, but they are invoked differently ... instead of CI3's
-  ``$this->load->view(x);``, you can use ``return view(x);``.
+  ``$this->load->view('x');``, you can use ``return view('x');``.
 - CI4 supports *View Cells* to build your response in pieces, and *View Layouts* for page layout.
 - The template parser is still there, and substantially enhanced.