refine: avoid full Func clone and fix comment typo in resolver

[prql-bot]

Two small cleanups in semantic/resolver/functions.rs, found in the nightly code-quality sweep.

• expr_of_func: func.clone().body.ty deep-cloned the entire Func (params, args, body, env, name hints) only to read body.ty, then discarded the clone. Replaced with func.body.ty.clone(), which clones just the Option<Ty>. Behavior is identical — func itself is still moved into ExprKind::Func(func) a few lines later, so nothing else changes.
• Doc comment typo: "Will return Err(func) is partial application is required" → "if partial application is required".

No regression test is included: the clone change is a pure allocation optimization that yields the same value, and the other change is a comment fix. Both are covered by the existing resolver test suite (cargo test -p prqlc --lib resolver, all passing).