Fix some typos detected by Debian's lintian QA tool

I assume the demoninator is a typo, rather than an indication that
someone has been playing too much Doom :-)

Signed-off-by: Simon McVittie <smcv@collabora.com>

include/SDL3/SDL_render.h

@@ -2747,7 +2747,7 @@ extern SDL_DECLSPEC bool SDLCALL SDL_RenderDebugText(SDL_Renderer *renderer, flo
  * Draw debug text to an SDL_Renderer.
  *
  * This function will render a printf()-style format string to a renderer.
- * Note that this is a convinence function for debugging, with severe
+ * Note that this is a convenience function for debugging, with severe
  * limitations, and is not intended to be used for production apps and games.
  *
  * For the full list of limitations and other useful information, see