Some links broken for Project Wiki

[Devin-64]

Due to the reason commit and adding a additional folder to the src hierarchy links in the project Wiki now have errors. For PolicyModification-and-PolicyQuery.md, Custom-serialization.md, and Custom-PAP.md the links are now broken as a "/core" is missing after the "https://github.com/usnistgov/policy-machine-core/blob/master/src/main/java/gov/nist/csd/pm/...". Additionally for Explain-on-PDP-methods.md the link works but only by specifying a preexisting release and thus no longer following the formatting consistent across all other links "https://github.com/usnistgov/policy-machine-core/blob/04fdd91558918403199bdb18efa0fd660f3529f5/..."