Update PCA.md

Co-authored-by: Gregor Gorjanc <gregor.gorjanc@gmail.com>

Author: hannesbecher

PCA.md

@@ -124,7 +124,7 @@ gtSvd = np.linalg.svd(gtMat - gtMat.mean(axis=0), full_matrices=False)
 To demonstrate that branch PCA works without variant data, we run it on the ARG without mutations, `ts`.
 
 ```{code-cell} ipython3
-# haplotypes, each sample haplotype is ues by default
+# haplotypes, each sample haplotype is used by default
 hapBranchPca=ts.pca(num_components=10)
 ```