include: Fix a preformatted piece of documentation that isn't C code.

include/SDL3/SDL_surface.h

@@ -875,7 +875,7 @@ extern DECLSPEC int SDLCALL SDL_FillSurfaceRects(SDL_Surface *dst, const SDL_Rec
  * The blit semantics for surfaces with and without blending and colorkey are
  * defined as follows:
  *
- * ```c
+ * ```
  *    RGBA->RGB:
  *      Source surface blend mode set to SDL_BLENDMODE_BLEND:
  *       alpha-blend (using the source alpha-channel and per-surface alpha)