[Chore] README.md: Adjusted the 'Phong Reflection Model'

Previously, the markdown table wasn't using equal sizing of the
cells on smaller screens. This change unifies the usage of the
hyphen cell delimiter and switches to '<img>' tags to control
the width of each image in the rendered cell.

README.md

@@ -12,9 +12,9 @@ It loosely follows the steps described in [The Ray Tracer Challenge](http://rayt
 
 ### Phong Reflection Model
 
-| Ambient Reflection| Diffuse Reflection | Specular Reflection | Result |
-| :----------------:| :----------------: | :-----------------: | :----: |
-| ![Ambient Reflection of a sphere](../assets/readme/phong-shading-01-ambient-reflection.png?raw=true) | ![Diffuse Reflection of a sphere](../assets/readme/phong-shading-02-diffuse-reflection.png?raw=true)| ![Specular Reflection of a sphere](../assets/readme/phong-shading-03-specular-reflection.png?raw=true) | ![Phong Reflection Render Result of a sphere](../assets/readme/phong-shading-04-result.png?raw=true) |
+| Ambient Reflection  | Diffuse Reflection  | Specular Reflection |       Result        |
+| :-----------------: | :-----------------: | :-----------------: | :-----------------: |
+| <img src="../assets/readme/phong-shading-01-ambient-reflection.png?raw=true" alt="Ambient Reflection of a sphere", width="220" /> | <img src="../assets/readme/phong-shading-02-diffuse-reflection.png?raw=true" alt="Diffuse Reflection of a sphere", width="220" /> | <img src="../assets/readme/phong-shading-03-specular-reflection.png?raw=true" alt="Specular Reflection of a sphere", width="220" /> | <img src="../assets/readme/phong-shading-04-result.png?raw=true)" alt="Phong Reflection Render Result of a sphere", width="220" /> | 
 
 ## Usage