fix/replace waybackmachine links

[wasita]

there were some broken links that i wound up replacing with the waybackmachine (i.e., cached/web archived) version, just so that the content could be preserved. if you want to replace them with different references entirely, here are the links that need to be replaced:

• content/GLM_Single_Subject_Model.ipynb: http://mindhive.mit.edu/node/116 (under filtering section)
• content/Preprocessing.ipynb: https://elef.soic.indiana.edu/documentation/0.15.0.dev/examples_built/syn_registration_2d/ (under spatial normalization section)