Remove DOI link that may generate 403 errors

Author: nanxstats

README.md

@@ -6,7 +6,7 @@
 [![Downloads from the RStudio CRAN mirror](https://cranlogs.r-pkg.org/badges/protr)](https://cranlogs.r-pkg.org/badges/protr)
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-Comprehensive toolkit for generating various numerical features of protein sequences described in Xiao et al. (2015) <[DOI:10.1093/bioinformatics/btv042](https://doi.org/10.1093/bioinformatics/btv042)> ([PDF](https://nanx.me/papers/protr.pdf)).
+Comprehensive toolkit for generating various numerical features of protein sequences described in Xiao et al. (2015) ([PDF](https://nanx.me/papers/protr.pdf)).
 
 ## Paper citation