Compiler error messages: reduce assertiveness of message E0384

This message is emitted as guidance by the compiler when a developer attempts to reassign a value to an immutable variable.  Following the message will always currently work, but it may not always be the best course of action; following the 'consider ...' messaging pattern provides a hint to the developer that it could be wise to explore other alternatives.

compiler/rustc_mir/src/borrow_check/diagnostics/conflict_errors.rs

@@ -1681,7 +1681,7 @@ impl<'cx, 'tcx> MirBorrowckCtxt<'cx, 'tcx> {
                 if decl.can_be_made_mutable() {
                     err.span_suggestion(
                         decl.source_info.span,
-                        "make this binding mutable",
+                        "consider making this binding mutable",
                         format!("mut {}", name),
                         Applicability::MachineApplicable,
                     );