docs(contrib): Fix steps to work on duplicate entries

The instructions previously instructed the contributor
to run `SNAPSHOTS=overwrite cargo test` which will however
fail when the added entry is duplicate with:

    thread 'codegen' panicked at 'Duplicate present: ...'

Duplicates have to be removed by firstly running the verify
test with SNAPSHOTS=overwrite before you should try running
the code generation.

These were actually the steps before they were
changed in 7eb8873950
(which appears to have been an oversight).

CONTRIBUTING.md

@@ -41,11 +41,11 @@ Otherwise, to add to the dictionary:
 
    Format: `typo,correction[,correction...]`
 
-2. Code-gen the dictionary
+2. Verify (and postprocess) the dictionary
 
    With `cargo` and `rustfmt` installed, run
    ```console
-   $ SNAPSHOTS=overwrite cargo test -p typos-dict
+   $ SNAPSHOTS=overwrite cargo test -p typos-dict verify
    ```
    (we do development-time code-gen to speed up builds)
    
@@ -54,11 +54,11 @@ Otherwise, to add to the dictionary:
    - Mixing up corrections and typos
    - etc
 
-3. Verify your change
+3. Code-gen the dictionary
 
    Run
    ```console
-   $ cargo test -p typos-dict
+   $ SNAPSHOTS=overwrite cargo test -p typos-dict codegen
    ```
 
 ### Process