fix: correct changelog spelling in header

[cristovaov]

In my opinion this is just a spelling mistake and not a breaking change.

[coveralls]

Coverage remained the same at 100.0% when pulling bca0b05 on cristovaov:patch-1 into aad6a61 on conventional-changelog:master.

[jbottigliero]

Going to link this to #299 for tracking, but this looks good to me.

[bcoe]

@cristovaov I kind of think this is a point of opinion, looking at:

https://keepachangelog.com/en/0.3.0/

They use the term Change Log throughout the site ... what if you instead refactor it so that you can pass in the header in configuration?

[cristovaov]

@bcoe I'd respectfully argue why I believe (or opine) change log to be a spelling mistake :)

They've passed to 1.0.0 for while now: https://keepachangelog.com/en/1.0.0/
where all references to "change log" were changed to "changelog" in that version.
I've looked up what was grammarly correct but admit that it's somewhat inconclusive dictionary wise:

1. The Free Dictionary:
   • points 'changelog' to their encyclopedia article and Wikipedia (as mentioned in #299) and quite extensive.
   • [points 'change log' has just one line and directs to see change file.
2. Mirriam-Webster has no entry for 'changelog' nor 'change log'.
3. Oxford Dictionary:
   • has no entry for 'changelog' but has one though for 'change log'
     However despite 3, changelog seems to be the accepted and correct spelling. Also see issue #92 on keep-a-changelog.

This said, I'm not sure if I'm knowledgeable enough to make the header configurable. I could give it a go at the weekend perhaps!

edit: fixed a sentence

[bcoe]

addressed in #335, you convinced me of the spelling 😄

[cristovaov]

Thank you @bcoe !