#390 doc/openocd.texi: typo "work are size"

[Citrine RainSlide]

In line 4985 of doc/openocd.texi, there is a typo "work are size" which should be "work area size" instead.

Since SourceForge don't support locating line, here are the links to line 4985 of doc/openocd.texi on the two mirrors:

• https://repo.or.cz/openocd.git/blob/880ae3f07744a09e267bde00cf405c85a6f10482:/doc/openocd.texi#l4985
• https://github.com/openocd-org/openocd/blob/561f27fde9dc51f014b672c227dc7c8ca2530fcf/doc/openocd.texi#L4985