Update PCA.md

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

Author: hannesbecher

PCA.md

@@ -121,7 +121,7 @@ gtSvd = np.linalg.svd(gtMat - gtMat.mean(axis=0), full_matrices=False)
 ```
 
 ## Branch PCA (tskit)
-To demstrate that branch PCA works without variant data, we run it on the ARG without mutations, `ts`.
+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