Doc: remove built-in from theme names

The string was hidden in the UI. This needs to come back
when the condition changes again.

Change-Id: I096fdbe0ff15f9dbaa7fb04a0dbb6271fab87315
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

doc/src/overview/creator-configuring.qdoc

@@ -102,8 +102,8 @@
     colors, and icons.
 
     To switch from the default theme to a dark theme, select \gui Tools >
-    \gui Options > \gui Environment, and then select \gui {Dark (built-in)} in
-    the \gui Theme field.
+    \gui Options > \gui Environment, and then select \gui Dark in the \gui Theme
+    field.
 
     You can use the \QC text and code editors with your favorite color scheme
     that defines how code