[trivial] Fix ddoc source path of rt_dmain2

[JohanEngelen]

The source path of many rt files is absent or broken. This fixes it for one file.

[dlang-bot]

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[wilzbach]

Thanks. Good catch!

[wilzbach]

Interestingly CircleCi fails consistently with:

****** FAIL debug64 core.time
core.exception.AssertError@src/core/time.d(527): unittest failure
----------------

Reproducible with:

TEST_COVERAGE=1 make -f posix.mak unittest-debug 

[wilzbach]

The CircleCi failure is clearly unrelated and I am working at #2048 to fix it. However as #2048 has no clear direction except that a single line is responsible and all other CIs are happy with this - I am going to pull this. It just changes the Ddoc comment after all.