Fix broken make target "realclean"

Without this patch I get the following error when running `make distclean`:
```
...
make -C ./documentation realclean 
make[1]: Entering directory '/home/marko/work/stripperFoilPlatter/src/parker6k/documentation'
make[1]: *** No rule to make target 'realclean'.  Stop.
make[1]: Leaving directory '/home/marko/work/stripperFoilPlatter/src/parker6k/documentation'
/etc/epics/configure/RULES_DIRS:70: recipe for target 'documentation.realclean' failed
make: *** [documentation.realclean] Error 2
```