small PR to normalize relative paths so it works on C++ side

[pachadotdev]

in the current codebase this works

dir <- "/home/pacha/Downloads"

tesseract_download("chi_sim_vert", datapath = dir, model = "best")
tesseract_download("chi_sim", datapath = dir, model = "best")

# compare the results: fast (text1) vs best (text2)
text1 <- ocr(file, engine = tesseract("chi_sim"))
text2 <- ocr(file, engine = tesseract("chi_sim", datapath = dir))

this won't

dir <- "~/Downloads"

tesseract_download("chi_sim_vert", datapath = dir, model = "best")
tesseract_download("chi_sim", datapath = dir, model = "best")

# compare the results: fast (text1) vs best (text2)
text1 <- ocr(file, engine = tesseract("chi_sim"))
text2 <- ocr(file, engine = tesseract("chi_sim", datapath = dir))

this PR fixes that