[region] fix dune file

src/plugins/region/dune

@@ -38,10 +38,8 @@
  (public_name frama-c-region.core)
  (flags -open Frama_c_kernel :standard -w -9)
  (libraries frama-c.kernel frama-c-server.core unionFind)
- (instrumentation
-  (backend landmarks))
- (instrumentation
-  (backend bisect_ppx)))
+ (instrumentation (backend landmarks))
+ (instrumentation (backend bisect_ppx)))
 
 (plugin
  (optional)