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

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