Prime Minister Menachem Begin has improved his standing with Israel voters in the campaign before general elections June 30. In a public opinion poll, 39.8 percent of respondents chose Mr. Begin as the candidate best suited to be Prime Minister. Opposition Labor Party leader Shimon Peres, who just a few months ago seemed to be heading for a landslide victory, was chosen by only 23 percent. The survey was held before last week's Israeli air strike against an Iraqi nuclear plant.