[Ada] Minor change to comment in System.Regexp spec

[Pierre-Marie de Rodat]

Fix the comment about the form of a 'term'. Allow a sequence of elmts,
in {...}, but not nested {...}.

Tested on x86_64-pc-linux-gnu, committed on trunk

2020-06-03  Bob Duff  <duff@adacore.com>

gcc/ada/

	* libgnat/s-regexp.ads: Fix comment
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