[tools/rexm] Update nextCatIndex

[CrackedPixel]

taken from #5612

i am less familiar with this tool so please advise 🙏 i don't know what the purpose of the +1 was

category list:
  [0]: core
  [1]: shapes
  [2]: textures  <-- nextCatIndex
  [3]: text      <-- +1
  [4]: models
  [5]: shaders

when searching for "textures" it was adding +1 and trying to affect "text", which messed up the generated output

it also crashes when running rexm validate (tested on linux) with current master with a segfault

with the PR changes it detects all categories perfectly and the crash doesn't happen anymore

[maiconpintoabreu]

Hi @CrackedPixel, the propose of the +1 is to find the next category and add the new example on the last position of the category pushing the next one down 1.

Ps. This fix will work untill you need to add a text example

[maiconpintoabreu]

I suggest to keep the +1 offset, otherwise it will add on the beginning of the example list.

[raysan5]

@CrackedPixel @maiconpintoabreu thanks for finding and fixing this issue!