docs: update nipype dependency

Author: oesteban

docs/requirements.txt

@@ -1,7 +1,7 @@
 git+https://github.com/AleksandarPetrov/napoleon.git@0dc3f28a309ad602be5f44a9049785a1026451b3#egg=sphinxcontrib-napoleon
 git+https://github.com/rwblair/sphinxcontrib-versioning.git@39b40b0b84bf872fc398feff05344051bbce0f63#egg=sphinxcontrib-versioning
 nbsphinx
-git+https://github.com/nipy/nipype.git@master#egg=nipype
+nipype >= 1.5.1
 nitransforms >= 20.0.0rc3,<20.2
 packaging
 pydot>=1.2.3