Updated README Cmake relative paths to current directory

[benjitrosch]

The current README's Cmake commands have relative paths that go up a directory with .. instead of using the current directory ., which seems to imply that the user is running the command while inside a folder—which is likely not the case.

Also, this would make the instructions match with https://github.com/xiph/vorbis.

[rillian]

Thanks for the patch.

Just above the first line you changed, the instructions say

    mkdir build
    cd build

So I think it's correct to write the following commands as if srcdir is one level above. Moreover, I thought using separate build directories was recommended practice for cmake?

One can stay in the source directory and pass the build directory on the command-line each time, but the options are inconsistent between cmake, cmake --build, and ctest so I find changing directory easier to follow.

[benjitrosch]

Hmm, I can't say what I was thinking 2 years ago, but you are 100% correct. I'll close the PR, thanks!