test: lint errors with vale

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

Author: dvdksn

content/build/building/context.md

@@ -207,7 +207,7 @@ clone a specific branch, tag, and subdirectory of a repository.
 
 The format of the URL fragment is `#ref:dir`, where:
 
-- `ref` is the name of the branch, tag, or remote reference
+- `ref` is the name of the branch, tag, or commit hash
 - `dir` is a subdirectory inside the repository
 
 For example, the following command uses the `container` branch,
@@ -231,6 +231,15 @@ contexts:
 | `myrepo.git#mytag:myfolder`    | `refs/tags/mytag`             | `/myfolder`        |
 | `myrepo.git#mybranch:myfolder` | `refs/heads/mybranch`         | `/myfolder`        |
 
+When you use a commit hash as the `ref` in the URL fragment, use the full,
+40-character string SHA-1 hash of the commit. A short hash, for example a hash
+truncated to 7 characters, is not supported.
+
+```text
+✅ 05d2d74403d23d99daf70f4eb4fc0ff16cae7f1a
+❌ 05d2d74
+```
+
 #### Keep `.git` directory
 
 By default, BuildKit doesn't keep the `.git` directory when using Git contexts.