BUG: cannot set expression from command-line (fixes #2012)

applications/utilities/preProcessing/setExprFields/setExprFields.C

@@ -5,7 +5,7 @@
     \\  /    A nd           | www.openfoam.com
      \\/     M anipulation  |
 -------------------------------------------------------------------------------
-    Copyright (C) 2019-2020 OpenCFD Ltd.
+    Copyright (C) 2019-2021 OpenCFD Ltd.
 -------------------------------------------------------------------------------
 License
     This file is part of OpenFOAM.
@@ -771,7 +771,7 @@ int main(int argc, char *argv[])
             expressions::exprString
                 expression
                 (
-                    args[expression],
+                    args["expression"],
                     dictionary::null
                 );