Fix microscopic typo in Core spec

[tiangolo]

Fix microscopic typo in Core spec.

Change

[...] an pre-existing [...]

to

[...] a pre-existing [...]

References:

• Webster's (very verbose article): https://www.merriam-webster.com/words-at-play/is-it-a-or-an
• Simpler/quicker article by a less known source: https://www.englishclub.com/pronunciation/a-an.htm

[handrews]

Thanks!