docs: correct missing uppercase in Markdown files

To detect these typos I used:

```
clear && grep -rn '\. [a-z]' . | uniq | grep -v '\. lib' | grep -v '[0-9]\. [a-z]' | grep -v '\.\. [a-z]' | grep -v '\. curl' | grep -v 'e.g. [a-z]' | grep -v 'eg. [a-z]' | grep -v '\etc. [a-z]' | grep -v 'i.e\. [a-z]' | grep --color=always '\. [a-z]' | grep '\.md'
```

Closes #9474

docs/BUGS.md

@@ -245,7 +245,7 @@
  tracker.
 
  The `TODO` document is full of ideas and suggestions of what we can add or
- fix one day. you are always encouraged and free to grab one of those items and
+ fix one day. You are always encouraged and free to grab one of those items and
  take up a discussion with the curl development team on how that could be
  implemented or provided in the project so that you can work on ticking it odd
  that document.