Fix a little typo and wording

c/sdl/.gitignore

/hello_sdl
*.ttf
/zig-cache

c/sdl/README.md

![screenshot of hello\_sdl running](./screenshot.png)
And for everything else, see [`hello_sdl.c`](./hello_sdl.c).  To run it
run `make` and then run `./hello_sdl`.  Do not that you'll need a TTF
font from somewhere and then run it like this:
compile it by running make `make` and then run `./hello_sdl`.  Do note
that you'll need a TTF font from somewhere and then run it like this:
```
./hello_sdl /usr/share/fonts/TTF/DejaVuSerif.ttf