Fix command for manually executing unit testing

[pothitos]

Hello, Radovan, and thank you for the good project!

The previous command make test in the README.md doesn't work.

$ make test
Running tests...
Test project /home/npothito/repos/gtest-demo-gitlab/build
    Start 1: unit
1/1 Test #1: unit .............................   Passed    0.00 sec

100% tests passed, 0 tests failed out of 1

Total Test time (real) =   0.00 sec

[coveralls]

Coverage remained the same at 66.667% when pulling 64e7db6 on pothitos:patch-1 into 41a0b72 on bast:master.

[bast]

Thanks Nikolaos - actually it does work but does not show the output from the underlying unit test script. I will improve the doc.

[bast]

The doc is hopefully better now.