[fix]: fix bugs in task5

Author: chenjr97

mkdocs.yml

@@ -132,3 +132,6 @@ nav:
       - API 速查: task4_doc/apidoc.md
     - Task5:
       - 整体介绍: task5_doc/overview.md
+      - RV64 指令集参考: task5_doc/riscv.md
+      - 填空导读: task5_doc/guide.md
+      - API 速查: task5_doc/apidoc.md