docs: Update name of pfe-colors mixin in docs (#744)

Author: starryeyez024

docs/content/theme/writing-web-component-styles.md

@@ -78,7 +78,7 @@ Inside the stylesheet for a container component, the following snippet will allo
   $LOCAL: band;
 
   // Pull in pfe-color settings for all supported surface colors
-  @include surfaces;
+  @include pfe-surfaces;
 ```
 
 This mixin will compile to the following CSS, one entry for each supported surface color (darkest, darker, base, lighter, lightest, accent, complement):
@@ -291,4 +291,4 @@ If the container allows changes to background colors, then it should also influe
     --pfe-theme--color--surface--accent: #ee0000;
     ```
 
-    2. And every time you hit a variable as the value, [change it to a funny color](http://www.giphy.com/gifs/hu1kdgZ1ObIGPcENJ0), so you can verify that is indeed the property you are looking for.
+    2. And every time you hit a variable as the value, [change it to a funny color](http://www.giphy.com/gifs/hu1kdgZ1ObIGPcENJ0), so you can verify that is indeed the property you are looking for.