[update] Fixed a typo in README.md

On branch master
Untracked files:
	Bolgarsko-russkiy-slovar-izobrazitelnogo-iskusstva-Ivanka-Atanasova.pdf
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On branch master
Untracked files:
	Bolgarsko-russkiy-slovar-izobrazitelnogo-iskusstva-Ivanka-Atanasova.pdf
nothing added to commit but untracked files present (use "git add" to track)