feat: fix formatting block (tari-project#5630)

Description
---
Fixed the formatting block in
`base_layer/mmr/src/sparse_merkle_tree/mod.rs`

This caused a formatting warp, but sufficiently excluded the text from
doc tests:
```
//! .           ┌──────┐
```

This fixed the formatting warp, but added the text to the doc test,
which failed:
```
//!             ┌──────┐
```
Test error:
```
as
---- src\sparse_merkle_tree\mod.rs - sparse_merkle_tree (line 28) stdout ----
error: unknown start of token: \u{250c}
 --> src\sparse_merkle_tree\mod.rs:29:1
  |
3 | ┌──────┐
  | ^
```

This fixed both issues:
```
//! r###"                     ┌──────┐                            "###
```

Motivation and Context
---
See above

How Has This Been Tested?
---
Ran `rust update`, then `cargo +nightly fmt --all` and then `cargo test
--release`

What process can a PR reviewer use to test or verify this change?
---
Run `rust update`, then `cargo +nightly fmt --all` and then `cargo test
--release`

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Breaking Changes
---

- [x] None
- [ ] Requires data directory on base node to be deleted
- [ ] Requires hard fork
- [ ] Other - Please specify

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base_layer/mmr/src/sparse_merkle_tree/mod.rs

@@ -25,21 +25,21 @@
 //! you will notice that they the first two diverge at the first bit, while the first and last pairs differ at the
 //! fourth bit. This results in a SMT that looks like this:
 //!
-//!             ┌──────┐
-//!       ┌─────┤ root ├─────┐
-//!       │     └──────┘     │
-//!      ┌┴┐                ┌┴┐1
-//!   ┌──┤ ├──┐          ┌──┤ ├───┐
-//!   │  └─┘  │          │  └─┘   │
-//!  ┌┴┐     ┌┴┐        ┌┴┐10    ┌┴┐11
-//!  │0│  ┌──┤ ├──┐     │0│    ┌─┤ ├─┐
-//!  └─┘  │  └─┘  │     └─┘    │ └─┘ │
-//!      ┌┴┐     ┌┴┐       110┌┴┐   ┌┴┐111
-//!    ┌─┤ ├─┐   │0│          │0│ ┌─┤ ├─┐
-//!    │ └─┘ │   └─┘          └─┘ │ └─┘ │
-//!   ┌┴┐   ┌┴┐                  ┌┴┐   ┌┴┐
-//!   │A│   │B│                  │D│   │C│
-//!   └─┘   └─┘                  └─┘   └─┘
+//! r###"                     ┌──────┐                            "###
+//! r###"               ┌─────┤ root ├─────┐                      "###
+//! r###"               │     └──────┘     │                      "###
+//! r###"              ┌┴┐                ┌┴┐1                    "###
+//! r###"           ┌──┤ ├──┐          ┌──┤ ├───┐                 "###
+//! r###"           │  └─┘  │          │  └─┘   │                 "###
+//! r###"          ┌┴┐     ┌┴┐        ┌┴┐10    ┌┴┐11              "###
+//! r###"          │0│  ┌──┤ ├──┐     │0│    ┌─┤ ├─┐              "###
+//! r###"          └─┘  │  └─┘  │     └─┘    │ └─┘ │              "###
+//! r###"              ┌┴┐     ┌┴┐       110┌┴┐   ┌┴┐111          "###
+//! r###"            ┌─┤ ├─┐   │0│          │0│ ┌─┤ ├─┐           "###
+//! r###"            │ └─┘ │   └─┘          └─┘ │ └─┘ │           "###
+//! r###"           ┌┴┐   ┌┴┐                  ┌┴┐   ┌┴┐          "###
+//! r###"           │A│   │B│                  │D│   │C│          "###
+//! r###"           └─┘   └─┘                  └─┘   └─┘          "###
 //!
 //! The merkle root is calculated by hashing nodes in the familiar way.
 //! ```rust