Since pedantry tends to be contagious, I want to point out that this is a highly divisive issue. In fact, if we take into account grammar books, while it is acknowledged that the apostrophe is used to express omission of letters, this is only allowed in very specific environments. One of which, I'm afraid, is not the plural of nouns, or abbreviations. In fact, you shouldn't use the apostrophe to form plurals, except when it comes to single letters (the p's, the o's), since it easily leads to ambiguities.
Having said that, as with many other things in English, there is not unanimity about this, and there are people who claim that using the apostrophe is fine even in those circumstances. Furthermore, some stylebooks include rules to that effect. In any case, the prevalent tendency among grammarians is still to recommend avoiding the apostrophe.
For those interested in the matter, a more comprehensive explanation can be found here:
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PS: Sorry for the grammar nazism, but I teach English and couldn't help myself.