Fix Grammar in example-formula.rb

Two changes in comments: its -> it's
No source code was changed.

Before:
```
# This is a non-functional example formula to showcase all features and
# therefore, its overly complex and dupes stuff just to comment on it.
# You may want to use `brew create` to start your own new formula!
# Documentation: https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook
```

```
# version is seldom needed, because its usually autodetected from the URL/tag.
```

After:
```
# This is a non-functional example formula to showcase all features and
# therefore, it's overly complex and dupes stuff just to comment on it.
# You may want to use `brew create` to start your own new formula!
# Documentation: https://github.com/Homebrew/homebrew/wiki/Formula-Cookbook
```

```
# version is seldom needed, because it's usually autodetected from the URL/tag.
```