"an URL" is grammatically incorrect, as "URL" is pronounced "you-are-el". "An" should only be used when the first sound of the next word begins with a vowel sound, rather than a consonant sound.

This issue is present a few times in schema.rdfa, specifically at Line 4591, Line 4563, and Line 10581.

Sources for the grammar rule:

• American Psychological Association Style Guide
• Chicago Manual of Style
• English Stack Exchange