It isn't an apostrophe, it's a marker of secondary stress, showing that the secondary stress is on the "mju" syllable (the primary stress being on "kei"). Somewhere in the book that you're using there should be a key showing what all the pronunciation symbols mean.
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It isn't an apostrophe, it's a marker of secondary stress, showing that the secondary stress is on the "mju" syllable (the primary stress being on "kei"). Somewhere in the book that you're using there should be a key showing what all the pronunciation symbols mean.