exercises(all-your-base): example: fix for Zig 0.12.0-dev (#367)

Resolve this error, which occurs with the latest Zig master release:

    $ zig version
    0.12.0-dev.2341+92211135f
    $ bin/run-tests all-your-base
    Running tests for all-your-base...
    all_your_base.zig:30:9: error: local variable is never mutated
        var result = try list.toOwnedSlice();
            ^~~~~~
    all_your_base.zig:30:9: note: consider using 'const'

exercises/practice/all-your-base/.meta/example.zig

@@ -27,7 +27,7 @@ fn fromBase10(allocator: mem.Allocator, num: u32, output_base: u32) mem.Allocato
         n /= output_base;
     }
     if (list.items.len == 0) try list.append(0);
-    var result = try list.toOwnedSlice();
+    const result = try list.toOwnedSlice();
     mem.reverse(u32, result);
     return result;
 }