docs: Corrected article usage Update erc20-supply.adoc

[0xbryer]

The text contained a minor grammatical inconsistency in the article usage before "ERC-20." The phrase:

"We've seen how to implement a ERC-20 supply mechanism..."

was corrected to:

"We've seen how to implement an ERC-20 supply mechanism..."

This change ensures proper article usage, as "ERC-20" starts with a vowel sound.

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