[Ada] Minor reformatting of comments and some code, plus a typo correction

[Ada] Minor reformatting of comments and some code, plus a typo correction

gcc/ada/

	* exp_ch6.adb: Add a comma and fix a typo (machinary =>
	machinery) in comment.
	* exp_aggr.adb: Reformat, fix capitalization, and add a couple
	of commas in a comment. Adjust columns in several code
	fragments.
	* sem_aggr.adb: Reformat and add a comma in a comment.