Update README.md

Changed variable called "spacing" to "padding" in the variable declaration section as the variable "padding" is what's actually used throughout the exercise.

Author: dylanbyars

exercises/03 - CSS Variables/README.md

@@ -33,7 +33,7 @@ The purpose of this exercise is to gain experience using _CSS3 variables_. These
       :root {
         --base: #ffc600;
         --blur: 10px;
-        --spacing: 10px;
+        --padding: 10px;
       }
       ```
   2. Declare a new style for the `img` element and set the `background`, `filter`, and
@@ -65,4 +65,4 @@ The purpose of this exercise is to gain experience using _CSS3 variables_. These
       they require a suffix. We can use this to attach the correct suffix to the
       value if necessary.
 
-Wooooooo! Finished yaaaaay!
+Wooooooo! Finished yaaaaay!