Fix bug #342.

The Mercury implementation of string formatting (used by the non-C grades) was
incorrectly formatting some numbers when using the 'g' conversion specifier,
for example:

    string.format("%6.3g\n", [f(16.0)]) = "10"

The cause was an off-by-one error introduced when calls to string.substring
were changed to calls string.between (in commit
b1af59cb29ac31db9fc5b3422fced505facabad1).

library/string.m:
	Fix the off-by-one error above.

NEWS:
	Announce the fix.