Fix shaders path

[ghost]

Adds the ApplicationDirectory path to the shader files path of the shaders_mesh_instancing.c (R71-R73) example.

Fixes #3315.

Very likely also fixes "REVIEW: Example: models_mesh_instancing. Not working properly on web, issues with the shader but ONLY when uploade to GitHub, it works as expected locally." from #2952.

Edit: added example reference and line marks.

[raysan5]

@ubkp I'm afraid this is not the issue. If that was the issue, no shaders example would work, all follow the same loading pattern.

I just tried the example on desktop and web (locally) and it works as expected with latest raylib at this same moment.

Attaching my local compilation: shaders_mesh_instancing.zip

I vaguely remember I already faced this issue in the past but it's really weird, it works ok locally but once uploaded to GitHub it just stops working, issue is related to shader line-breaks (LF vs CR LF) but I don't know why it only fails for the specific shaders used in this example.

[ghost]

@raysan5 Yes, you're right. I've jumped into the first thing that appeared to be a solution. I should review these more carefully. My bad, sorry.