Bring docs agda version in sync with .cabal file

[liesnikov]

Docs say you need 2.6.3, cabal file says you need 2.6.4.

agda2hs/agda2hs.cabal

Lines 49 to 50 in b269164

	build-depends: base >= 4.10 && < 4.20,
	Agda >= 2.6.4 && < 2.6.5,

(the code above is from v1.2 tag, but github refuses to render it if it's not a permalink)

[jespercockx]

Thanks!