fix improper markdown code block indenting

As per the TODO:

- [ ] Verify the markdown code blocks are properly indented

```sh
find . -name '*.md' ! -name markdown.md -print0 | xargs -0 pcregrep -M '^``.*\n[^|`\n \t]'
```

Fixed `1998/dorssel/dorssel.md` and `thanks-for-help.md`.

Performed `make quick_www` to process the above.

thanks-for-help.md

@@ -3674,6 +3674,7 @@ changed. As was noted in the log at the time:
 >    (but obviously it's a terrible idea) to redefine main() (!!) so that any
 >    program that uses SDL1 has to have the same args as their definition.
 >    This program had 'main()' so the error message was:
+
 ``` <!---c-->
     thadgavin.c:60:1: error: conflicting types for 'SDL_main'
     main()
@@ -3687,6 +3688,7 @@ changed. As was noted in the log at the time:
     1 warning and 1 error generated.
     make: *** [thadgavin_sdl] Error 1
 ```
+
 >
 >    Thus `main()` was changed to `int main(int argc, char **argv)`.