wikiheaders.pl turns SDL_ into links even when already a link

[weslord]

Markdown links of the form [SDL_somestring](https://someurl) get transformed into [[SDL_somestring](SDL_somestring)](https://someurl), which is rendered on the wiki as a broken link followed by the full url.

Here's how I stumbled upon it:

1. https://github.com/libsdl-org/sdlwiki/commit/06194f7006cbdaadc41ea583f6796c6bf8e53e7e Edited the wiki to fix a link
2. https://github.com/libsdl-org/SDL/commit/d283f4651c94dcf15c159963bfeacba45cf994bb Markdown-formatted link saved to the header file
3. https://github.com/libsdl-org/sdlwiki/commit/f65040229a967c023421a9b8d8dfcc5bf247ea05 Header-file link turned back into two links on the wiki

Looks like there's one other example of this here: https://github.com/libsdl-org/sdlwiki/blob/ba4f2bfe8a73fa31a49989b6d77c71bcd2fc433c/SDL3/CategoryGPU.md?plain=1#L119