docs: Update structural-directives.md

[harishkrishnan24]

A Grammatical error in the section - "Type narrowing with template guards"

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Other information

[JeanMeche]

Thanks for this !

[JeanMeche]

Thanks for this !

[JeanMeche]

Could you please squash your commits into a single one. Thank you.

[JeanMeche]

You added 2 commits with that merge, this PR should be a single commit.

[harishkrishnan24]

You added 2 commits with that merge, this PR should be a single commit.

yeah sorry. Fixing it up

[harishkrishnan24]

@JeanMeche done. Squashed the commits

[thePunderWoman]

This PR was merged into the repository by commit aad3c2d.

The changes were merged into the following branches: main, 18.0.x

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