Fix Issue 18602 - [Better C] docs (duplicate "Dynamic arrays")

> "Dynamic arrays" shows up in both "Retained Features" and "Not Available" sections

Author: wilzbach

spec/betterc.dd

@@ -74,7 +74,7 @@ $(H2 $(LNAME2 retained, Retained Features))
     $(LI Nested functions, nested structs, delegates and lambdas)
     $(LI Member functions, constructors, destructors, operating overloading, etc.)
     $(LI The full module system)
-    $(LI Dynamic arrays, array slicing, and array bounds checking)
+    $(LI Array slicing, and array bounds checking)
     $(LI RAII (yes, it can work without exceptions))
     $(LI `scope(exit)`)
     $(LI Memory safety protections)