fix: clean up spacing in PREVRANDAO documentation

[pengqima]

Remove accidental extra spaces from the PREVRANDAO stack output description.

This changes “previous block's RANDAO mix” to “previous block's RANDAO mix”, improving readability and keeping the documentation formatting consistent.

[vercel]

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[cursor]

PR Summary

Low Risk
Documentation-only wording/spacing change with no runtime or API impact.

Overview
Fixes double spaces in the PREVRANDAO (44_merge.mdx) stack output line so it reads “previous block's RANDAO mix” instead of “previous block's” and “RANDAO” with extra gaps. No behavioral or opcode semantics change—documentation formatting only.

^{Reviewed by Cursor Bugbot for commit 74b5122. Configure here.}