[spec] fix changelog path in release job

The release step was using an absolute path (`src/...`) but the
working directory is already `src`, resulting in a wrong path
(`src/src/...`).

Author: i4004

.github/workflows/build.yml

@@ -56,7 +56,7 @@ jobs:
       - name: Extract changelog entry
         run: |
           awk '/^## \[/{if(f) exit; f=1; next} f' \
-            src/Simplify.Web.MessageBox/CHANGELOG.md \
+            Simplify.Web.MessageBox/CHANGELOG.md \
             > /tmp/release_notes.md
 
       - name: Create release