UI: Fix typo in dark theme causing wrong borders

This fixes an issue where the borders for certain types of windows would
not match the intended border style/color.  It was supposed to be 1
through 6 for frameShape, but I ended up putting 5 twice.

obs/data/themes/Dark.qss

@@ -37,7 +37,7 @@ QWidget {
     selection-color: black;
 }
 
-* [frameShape="1"], * [frameShape="2"], * [frameShape="3"], * [frameShape="4"], * [frameShape="5"], * [frameShape="5"] {
+* [frameShape="1"], * [frameShape="2"], * [frameShape="3"], * [frameShape="4"], * [frameShape="5"], * [frameShape="6"] {
     border: 1px solid rgb(31,30,31);
 }