rst2man doesn't have a suffix on Debian

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -167,7 +167,7 @@ if(EXISTS "${CMAKE_SOURCE_DIR}/machine/m
 endif(EXISTS "${CMAKE_SOURCE_DIR}/machine/m_${MACHINE}.man.rst")
 file(APPEND ${CMAKE_BINARY_DIR}/pg_top.1 "${MANUAL_SUPPLIMENT}")
 execute_process(
-    COMMAND rst2man.py ${CMAKE_BINARY_DIR}/pg_top.1.rst
+    COMMAND rst2man ${CMAKE_BINARY_DIR}/pg_top.1.rst
     OUTPUT_FILE ${CMAKE_BINARY_DIR}/pg_top.1
 )
 
