doc/coding-conventions: Fix version attribute suffix to match reality

The current doc is wildly out of touch with reality. A regex search shows
the following stats.

```
Style example  Frequency  Regex used
nix-2-5:            8     [a-zA-Z]-[0-9]+(-[0-9]+)+ =
nix-2_5:           17     [a-zA-Z]-[0-9]+(_[0-9]+)+ =
nix_2_5:          689     [a-zA-Z]_[0-9]+(_[0-9]+)+ =
nix_2-5:            1     [a-zA-Z]_[0-9]+(-[0-9]+)+ =
```

1 files changed, 1 insertions, 1 deletions

diff --git a/doc/contributing/coding-conventions.chapter.md b/doc/contributing/coding-conventions.chapter.md
index 79d90c23a40..cfe8582e514 100644
--- a/doc/contributing/coding-conventions.chapter.md
+++ b/doc/contributing/coding-conventions.chapter.md
@@ -224,7 +224,7 @@ There are a few naming guidelines:
 
 - Dashes in the package name _should_ be preserved in new variable names, rather than converted to underscores or camel cased — e.g., `http-parser` instead of `http_parser` or `httpParser`. The hyphenated style is preferred in all three package names.
 
-- If there are multiple versions of a package, this _should_ be reflected in the variable names in `all-packages.nix`, e.g. `json-c-0-9` and `json-c-0-11`. If there is an obvious “default” version, make an attribute like `json-c = json-c-0-9;`. See also [](#sec-versioning)
+- If there are multiple versions of a package, this _should_ be reflected in the variable names in `all-packages.nix`, e.g. `json-c_0_9` and `json-c_0_11`. If there is an obvious “default” version, make an attribute like `json-c = json-c_0_9;`. See also [](#sec-versioning)
 
 ## File naming and organisation {#sec-organisation}