fix: grammatical errors in template-syntax.md

[seafoamteal]

Description of Problem

"the double mustaches interprets"

Since "mustaches" is a plural noun, the verb "to interpret" must also be conjugated in its plural form.

"user-provided content"

There is a missing period/full stop at the end of this sentence.

"a SPA"

Since the pronunciation of SPA begins with a vowel sound, the article used should be "an", not "a".

Proposed Solution

1. Changed "interprets" to "interpret"
2. Added the period
3. Changed "a" to "an"

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[Cyebukayire]

I love the way you provided details for the changes. It's so professional

[NataliaTepluhina]

@defyentropy thank you for the contribution! Changes look good to me 👍🏻