fix: Update createPasskeySigner reference

[germartinez]

Context

This PR:

• Fixes the indentation of the return section in the reference.
• Adds the code snippet for the credential parameter of the function.

[github-actions]

Branch preview

✅ Deployed successfully in branch deployment:

https://fix_passkeys_reference--docs.review.5afe.dev

[github-actions]

Overall readability score: 31.62 (🟢 +0)

File	Readability
createpasskeysigner.mdx	60.13 (🟢 +0)

View detailed metrics

🟢 - Shows an increase in readability
🔴 - Shows a decrease in readability

File	Readability	FRE	GF	ARI	CLI	DCRS
createpasskeysigner.mdx	60.13	53.88	7.3	11	14	8.47
	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0

Averages:

	Readability	FRE	GF	ARI	CLI	DCRS
Average	31.62	27.51	13.66	17.37	15.71	8.96
	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0	🟢 +0

View metric targets

Metric	Range	Ideal score
Flesch Reading Ease	100 (very easy read) to 0 (extremely difficult read)	60
Gunning Fog	6 (very easy read) to 17 (extremely difficult read)	8 or less
Auto. Read. Index	6 (very easy read) to 14 (extremely difficult read)	8 or less
Coleman Liau Index	6 (very easy read) to 17 (extremely difficult read)	8 or less
Dale-Chall Readability	4.9 (very easy read) to 9.9 (extremely difficult read)	6.9 or less