ada: Fix various typos in GNAT RM

ada: Fix various typos in GNAT RM

List of unknown words in files can be produced with:

  $ cat *.rst | ispell -l |
    tr '[:upper:]' '[:lower:]' | sort | uniq | less

and can be easily filtered with eyes.

gcc/ada/

	* doc/gnat_rm/implementation_defined_aspects.rst: Fix typos.
	* doc/gnat_rm/implementation_defined_attributes.rst: Likewise
	* doc/gnat_rm/implementation_defined_characteristics.rst: Likewise
	* doc/gnat_rm/implementation_defined_pragmas.rst: Likewise
	* doc/gnat_rm/standard_library_routines.rst: Likewise.
	* gnat_rm.texi: Regenerate.