Custom_codelet.rst: Fixing TF user guide links to conform to new Poplar API structure

Summary:
Applies to TF1 and TF2.
The new structure for the Poplar API doc uses individual html files for each header file. The existing links have to be updated.

Reviewers: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, jamiep, markf

Reviewed By: #tensorflow, #framework_ip_review_-_any_oss_or_third-party_code_use_has_been_approved, markf

Subscribers: markf

Maniphest Tasks: T47189

Differential Revision: https://phabricator.sourcevertex.net/D53029

Author: jayniep-gc

tensorflow/compiler/plugin/poplar/docs/custom_codelet.rst

@@ -416,7 +416,7 @@ allocate the tensors with linear mapping.
 You can override this behaviour by defining a function that allocates
 tensors in a way that is most efficient for your operation.
 See the section on variable mapping in the `Poplar and PopLibs API Reference
-<https://docs.graphcore.ai/projects/poplar-api/page/poplar_api.html#poplar-variablemappingmethod-hpp>`_
+<https://docs.graphcore.ai/projects/poplar-api/page/poplar_api/poplar/graph/VariableMappingMethod.html>`_
 for moe information.
 
 To do this, define a function with the suffix ``_allocator`` with the following