README: should add `gen_step.step` when build `exe`

[bridgeQiao]

gen_step need build first:

diff --git a/README.md b/README.md
index fe39d36..997d3f0 100644
--- a/README.md
+++ b/README.md
@@ -44,6 +44,7 @@ pub fn build(b: *std.Build) !void {
     );
     exe.root_module.addImport("generated", gen_step.module);
     exe.root_module.addImport("flatbufferz", fbz_dep.module("flatbufferz"));
+    exe.step.dependOn(&gen_step.step);
 }