Hey Raja,
The [dspace-src] means the source code of the application, the one you fist installed DSpace from, in your case it's probably /build/dspace-4.2-src-release
The [dspace] directory refers to the compiled code, which is probably your /dspace directory. You shouldn't have to cd to this directory in order to perform the full rebuild.
One quick note tho:
I haven't used DSpace 4.2, only from 5.1 onward, so for versions 5.1, the documentation is a bit outdated (I think). The ant update step, where it refers to:
[dspace-source]/dspace/target/dspace-[version]-build.dir
It's actually referring to
[dspace-source]/dspace/target/dspace-installer
For version 4.2 maybe those instructions are accurate, however if you don't find the mentioned directory run the commands on the second directory.
Good luck,
Pedro Amorim