[HACKERS] Fix a typo in README.dependencies

Hi,

I found below formula to compute selectivities, but
I think the last Probability 'P(b=?)' should be 'P(c=?)'.

>  P(a=?,b=?,c=?) = P(a=?,b=?) * (d + (1-d)*P(b=?))

Attached patch fixes it, and it also adds some spaces
following another formula which is on line 86 and
computes P(a=?, b=?).

Regards,

--
Atsushi Torikoshi
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center

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