fix: proofread Set Theory II part

[vEnhance]

Proofreads the Set Theory II: Model Theory and Forcing part (issue #315), covering tex/set-theory/models.tex, tex/set-theory/forcing.tex, and tex/set-theory/CH.tex.

All findings are minor typos / notational slips; no mathematical arguments were rewritten.

models.tex — Reflection problem hint

• The hint quantified if there exists $a \in M$, but M is never defined in this problem (it's about $V_\kappa$). Changed to a \in V_\kappa, matching the solution.
• "use the fact that $\kappa$ is cofinal" → "$\kappa$ is regular". Cofinality isn't a standalone property of a cardinal here, and the solution itself invokes regularity.

forcing.tex

• Non-existence-of-generic example: $G \cap M = \varnothing$ → $G \cap D = \varnothing$. The point is that $G$ misses the dense set $D = \Po\setminus G$; $G \cap M$ is just $G$.
• Motivation for the forcing relation: $\left<\tau_r\right>$ → $\left<\tau, r\right>$ (missing comma in the pair).
• Definition of the forcing relation: "we write $\tau \Vdash \varphi$" → "$p \Vdash \varphi$" ($p \in \Po$ was just introduced; the body uses $p \Vdash$ throughout).
• Two phrasing fixes for consistency with the standard term: "dense below in $p$" → "dense below $p$", and "dense in $p$" → "dense below $p$".

CH.tex — countable chain condition

• $\Delta$-system lemma proof: "there exists an $a \in C$ contained in uncountably many $F \in C$" → "an element $a$ contained in …" ($a$ is a point that lies in uncountably many members, not itself a member of $C$).
• ccc proof of $\opname{Add}(\omega,\kappa)$: $B = \{p_\alpha \mid \dom(p_\alpha) \in R\}$ → \in \ol C (we keep the conditions whose domains lie in the $\Delta$-system $\ol C$; $R$ is the root, so $\dom(p_\alpha)\in R$ is a type error). Also clarified that $p_\alpha$ is restricted to $R$ before comparing (each $p_\alpha\restriction R \colon R \to {0,1}$), since finitely many such restrictions forces a collision.

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