Decrease too wide line in markdown (#8965)

In #8827 I wrote a line
which was longer than CI markdown linter allowed (max 120 chars, width
was 126)

This causes CI failure:
https://github.com/paritytech/polkadot-sdk/actions/runs/15850370092/job/44682093959?pr=8939

Author: Sajjon

docs/contributor/DOCUMENTATION_GUIDELINES.md

@@ -1,7 +1,7 @@
 # Documentation Guidelines
 
-This document is focused on documenting parts of the Polkadot SDK that relate to its external API. The list of such crates can
-be found in [CODEOWNERS](/.github/CODEOWNERS). Search for the crates auto-assigned to the `docs-audit` team.
+This document is focused on documenting parts of the Polkadot SDK that relate to its external API. The list of such
+crates can be found in [CODEOWNERS](/.github/CODEOWNERS). Search for the crates auto-assigned to the `docs-audit` team.
 
 These crates are used by external developers and need thorough documentation. They are the most concerned with FRAME
 development.