fixed friendly error system links

[lgilbuena]

• Fixed the links in https://beta.p5js.org/contribute/friendly_error_system/ regarding FES Message Generators

Addresses #1353

[Nwakaego-Ego]

Hi @lgilbuena, thank you for this PR. I tested this PR locally on the 2.0 branch and can confirm that the three links in the FES Message Generators section are correctly fixed. However, there is one remaining broken link. Line 174 was not updated:
[Dev Notes]: ./fes_contribution_guide.md#-development-notes. This still uses the old .md path and returns a 404 when clicked. It needs to be updated to match the same pattern as the other fixes:
[Dev Notes]: ../fes_contribution_guide/#-development-notes

Once that line is updated, this PR should be good to go.

CC @doradocodes @ksen0