Fix broken intra-doc links in the custom_hash example

[Edu-Amorim]

Description

cargo rbmt docs currently fails locally with the current cargo-rbmt:

error: unresolved link to `crate::accumulator::node_hash::BitcoinNodeHash`
 --> examples/custom_hash.rs:2:23
  |
2 | //! [BitcoinNodeHash](crate::accumulator::node_hash::BitcoinNodeHash), the one used by Bitcoin
  |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `accumulator` in module `custom_hash`
  |
  = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

error: unresolved link to `crate::accumulator::node_hash::NodeHash`
 --> examples/custom_hash.rs:4:30
  |
4 | //! implement the [NodeHash](crate::accumulator::node_hash::NodeHash) trait for it, and use it
  |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `accumulator` in module `custom_hash`

error: unresolved link to `crate::accumulator::MemForest::MemForest`
  --> examples/custom_hash.rs:12:56
   |
12 | //! This example shows how to use both the [MemForest](crate::accumulator::MemForest::MemForest) and
   |                                                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `accumulator` in module `custom_hash`

error: unresolved link to `crate::accumulator::node_hash::NodeHash`
  --> examples/custom_hash.rs:15:16
   |
15 | //! [NodeHash](crate::accumulator::node_hash::NodeHash) for your hash type.
   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `accumulator` in module `custom_hash`

error: could not document `rustreexo`
warning: build failed, waiting for other jobs to finish...
Error building docs: command exited with non-zero code `cargo --locked doc --all-features --no-deps --examples -p path+file:///home/edu/Documents/Apps/Projects/Neha/rustreexo#0.5.0`: 101

The doc links in examples/custom_hash.rs use crate::accumulator::... paths.
crate:: inside an example refers to the example binary, not to rustreexo, and the accumulator module was removed in #98, so the links can't resolve.

Verification

Verified by running cargo rbmt docs