fix ref formatting in paper

paper/paper.md

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 SourcePredict [(github.com/maxibor/sourcepredict)](https://github.com/maxibor/sourcepredict) is a Python package distributed through Conda, to classify and predict the origin of metagenomic samples, given a reference dataset of known origins, a problem also known as source tracking.
 
-DNA shotgun sequencing of human, animal, and environmental samples has opened up new doors to explore the diversity of life in these different environments, a field known as metagenomics [@metagenomics]. One aspect of metagenomics is investigating the community composition of organisms within a sequencing sample with tools known as taxonomic classifiers, such as Kraken @kraken.
+DNA shotgun sequencing of human, animal, and environmental samples has opened up new doors to explore the diversity of life in these different environments, a field known as metagenomics [@metagenomics]. One aspect of metagenomics is investigating the community composition of organisms within a sequencing sample with tools known as taxonomic classifiers, such as Kraken [@kraken].
 
 In cases where the origin of a metagenomic sample, its source, is unknown, it is often part of the research question to predict and/or confirm the source. For example, in microbial archaelogy, it is sometimes necessary to rely on metagenomics to validate the source of paleofaeces.
 Using samples of known sources, a reference dataset can be established with the taxonomic composition of the samples, *i.e.* the organisms identified in the samples as features, and the sources of the samples as class labels.