clarify shell dependencies

The contributed script uses bash features extensively and does not work with ksh.
diff --git a/BUILDING.txt b/BUILDING.txt
index 86fb45d..72aea76 100644
--- a/BUILDING.txt
+++ b/BUILDING.txt
@@ -158,8 +158,8 @@
 "legacy-friendly" version of the TigerVNC Server:
 
   cd {build_directory}
-  sh {source_directory}/contrib/xorg/build-xorg init
-  sh {source_directory}/contrib/xorg/build-xorg build [additional CMake flags]
+  bash {source_directory}/contrib/xorg/build-xorg init
+  bash {source_directory}/contrib/xorg/build-xorg build [additional CMake flags]
 
 build-xorg generates a version of Xvnc that has no external dependencies on the
 X11 shared libraries or any other distribution-specific shared libraries.  This