Improve clarity of comment

Author: Tarun Prabhu

flang/test/Driver/flang-f-opts.f90

@@ -27,8 +27,8 @@
 ! WARN-BUILTIN-MULTIPLE: warning: '-fbuiltin' is not valid for Fortran
 ! WARN-BUILTIN-MULTIPLE: warning: '-fno-builtin' is not valid for Fortran
 
-! When issuing a warning with a suggestion, ensure that the suggestion uses
-! "-Xflang" in the diagnostic message. This is typically issued when an option
+! When emitting an error with a suggestion, ensure that the diagnostic message
+! uses '-Xflang' instead of '-Xclang'. This is typically emitted when an option
 ! that is available for `flang -fc1` is passed to `flang`. We use -complex-range
 ! since it is only available for fc1. If this option is ever exposed to `flang`,
 ! a different option will have to be used in the test below.