fix extension for -Zdump-mir-dataflow graphviz files

Author: lqd

compiler/rustc_mir_dataflow/src/framework/engine.rs

@@ -285,7 +285,7 @@ where
 }
 
 Noneifdump_enabled(tcx,A::NAME, def_id) => {
-create_dump_file(tcx,".dot",false,A::NAME,&pass_name.unwrap_or("-----"), body)?
+create_dump_file(tcx,"dot",false,A::NAME,&pass_name.unwrap_or("-----"), body)?
 }
 
  _ => return(Ok(()), results),