concept/numbers: Update introduction.md

[lerontonge]

Improved readability of the first paragraph.

[petertseng]

I'm not sure. English isn't my first language, so I have a difficult time telling the difference between "Integers hold any number" and "An Integer holds any number". I need some help to understand what makes the latter clearer.

If the change is better, it should also be made to the following files which all have the same content. I don't like that multiple files have the same content and I have to ask contributors to make the same change in multiple places but apparently that is how it is.

• https://github.com/exercism/haskell/blob/main/exercises/concept/temperature/.docs/introduction.md
• https://github.com/exercism/haskell/blob/main/concepts/numbers/about.md

[kytrinyx]

This change feels clearer to me, @petertseng. My vote is to move forward with this PR.

@lerontonge Would you have the time to make the change in the other files mentioned as well?