Add a missing space to `README.md` and remove from table of contents the removed `Test vectors` section

[Benjamin-Loison]

The Test vectors section was forgot to be removed in https://github.com/chfast/ethash/commit/cdef24d186e3dfbf53fed75d1f276a12cedbe2cf#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5.

[codecov[bot]]

Codecov Report

Merging #232 (c7ca6f9) into master (e08bd0f) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #232   +/-   ##
=======================================
  Coverage   98.65%   98.65%           
=======================================
  Files          17       17           
  Lines        1337     1337           
=======================================
  Hits         1319     1319           
  Misses         18       18           

Flag	Coverage Δ
be	85.35% <ø> (ø)
default	98.41% <ø> (ø)
x86_64	85.03% <ø> (ø)

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[sonarqubecloud[bot]]

Kudos, SonarCloud Quality Gate passed!   

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