[Feature][30. Try Catch] Added `public` in try catch

[gsangeryee]

Details(细节)

Emphasize that try-catch can only be used for external and public function.

Motivation(动机)

Beginners may think that try catch can only be used in external functions.

Are you willing to submit a PR?(你愿意提交PR吗？)

• [X] Yes I am willing to submit a PR!(是的我愿意)

[phoouze]

The intended meaning is that try-catch can only be used to catch calls to external functions and the constructor function when creating a contract. This could lead to ambiguity in expression; it can be revised for clarity.

[XdpCs]

I will update it soon.