Fix typo in contributing guide.
Not sure how this got broken but it was probably an errant search and replace.
@@ -680,7 +680,7 @@ test/bin/none/pgbackrest help backup repo-type
To quickly view the HTML documentation, the `--no-exe` option can be passed to the documentation generator in order to bypass executing the code elements:
```
-pgbackrest/doc/doc.pl --output=html --no-exe
+pgbackrest/doc/doc.pl --out=html --no-exe
```
The generated HTML files will be placed in the `doc/output/html` directory where they can be viewed locally in a browser.
