docs: fix article typo in migration guide

[Rohan5commit]

Related issue: N/A (trivial docs grammar fix)

Fixes a grammar issue in the migration guide by changing "a old/new comparison" to "an old/new comparison".

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Pull request overview

Updates the migration guide to fix a grammar issue in the “Renamed files” section.

Changes:

• Corrected the article in the phrase “a old/new comparison” to “an old/new comparison”.