fix derivation of upper bound

Author: teorth

constants/27b.md

@@ -7,7 +7,7 @@ $C\_{27b}$ is the highest possible chromatic number for any biplanar graph.
 
 | Bound | Reference | Comments |
 | ----- | --------- | -------- |
-| 12 | Trivial [R1959] | Follows from the four color theorem. |
+| 12 | Trivial [R1959] | In fact, every biplanar graph has a vertex of degree at most 11.  |
 
 ## Known lower bounds