fix useless var

Unlike lists, this isn't mutated for maps because there's no
intermediate builder object being used for maps (lists are assembled
with an ArrayList, after which they are converted to plain slices).

src/zaye.zig

@@ -266,7 +266,7 @@ pub const Value = union(enum) {
         }
 
         fn endMap(self: *Builder, diag: *ParseDiagnostic) !void {
-            var top = self.container_stack.pop();
+            const top = self.container_stack.pop();
 
             if (top != .map) {
                 diag.setMessage("map ended when a map was not the top container");