doc(port_std): remove the false changelog entry

`r3_port_std` doesn't depend on `svgbobdoc`. This entry was accidentally
added in commit 3298ccf.

Author: yvt

src/r3_port_std/CHANGELOG.md

@@ -7,10 +7,6 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ## [Unreleased]
 
-### Fixed
-
-- Upgrade `svgbobdoc` to `^0.3.0` to fix build failures in documentation build
-
 ## [0.2.0] - 2022-03-15
 
 ### Changed