Fix the issue with examples in pcp function

[dolremi]

This PR fix one issue with the PCP function in notebook of Background Removal with Robust PCA.

• The examples array needs to get some random image of matrix S and L, therefore it should be examples.extend([S[:,140], L[:,140]]), where 140 is a random number. The original notebook is examples.extend([S[140,:], L[140,:]]), which will generate error when calling plots() function.