ada: Fix minor issues in -gnaty0's documentation

Before this patch, the documentation of -gnaty0 used 0-based indexing
for column numbers while 1-based indexing is used everywhere else. This
patch makes this documentation use 1-based indexing, and also adds a
missing parenthesis.

gcc/ada/

	* doc/gnat_ugn/building_executable_programs_with_gnat.rst: Fix
	minor issues.
	* gnat_ugn.texi: Regenerate.