meson doc: Is "-Dtap-tests" a typo?

Hi,

The parameter name "-Dtap-tests" in the meson's doc seems to be a typo.
I think "-Dtap_tests" is the correct name.  Attached patch fixes that.

I got an unknown options error by using "-Dtap-tests=enabled". However,
meson_options.txt told me the correct name was "-Dtap_tests".

regards,

-- 
Katsuragi Yuta
Advanced Computing Technology Center
Research and Development Headquarters
NTT DATA CORPORATION