Add missing `Proof.` to sealing proofs

This helps async proof checking (see
iris/iris!406 (comment 46759)).

Done with
```
gsed -i 's/seal \(.*\)\. by eexists. Qed./seal \1. Proof. by eexists. Qed./' \
  $(find theories/ -name '*.v')
```

And checked by inspecting the output of:
```
git grep '\bseal\b'|fgrep -v 'Proof. by eexists. Qed.'
```