Update staking.adoc

[saniksin]

Description of the Changes

Clarified a misleading instruction in the staking documentation.
The original text implied that validators must be attesting to blocks before staking.
However, this is incorrect - attestation is only possible after staking is completed and one full epoch has passed.

Original line:

a| * You should make sure you are xref:responsibilities[running a full node and attesting to blocks] before staking

Updated line:

a| * You should make sure you are xref:responsibilities[running a fully synchronized node] before staking. Attesting to blocks is only possible starting from the epoch following a successful stake.

This change helps avoid confusion for new validators who may attempt attestation too early.

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Check List

• Changes made against main branch and PR does not conflict
• PR title is meaningful
• Detailed description added under "Description of the Changes"
• Specific URL(s) added under "PR Preview URL"

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