add lambda to readme

Author: inishchith

python/readme.md

@@ -13,5 +13,5 @@
  8. [Sets](./08-Sets.md)
  9. [In build functions]()
  10. [User defined functions](./10-user_defined_functions.md)
- 11. [Lambda]()
+ 11. [Lambda](./11-Lambda.md)
  12. [Modules](./12-Modules.md)