fix(contributors): point projects.json at design-grammar (renamed from periodic-table)

a57c6d7eb9 renamed periodic-table → design-grammar but missed the
contributor-generator SSOT. The Update Contributors job inside
Book Validate was failing because projects.py update-files emitted
'periodic-table/.all-contributorsrc' and 'periodic-table/README.md'
which no longer exist, causing 'git add -A' to fail (silenced by
|| true) before any contributor files got staged. Subsequent
'git commit' had nothing to commit and exited 1 under set -e.

After this fix update-files emits design-grammar paths that exist.

.github/workflows/contributors/projects.json

@@ -78,13 +78,13 @@
             }
         },
         {
-            "key": "periodictable",
-            "dir": "periodic-table",
-            "aliases": ["periodic-table", "periodic_table", "periodic table"],
+            "key": "designgrammar",
+            "dir": "design-grammar",
+            "aliases": ["design-grammar", "design_grammar", "design grammar"],
             "section": {
                 "emoji": "⚗️",
-                "title": "Periodic Table of ML Systems Contributors",
-                "marker": "PERIODICTABLE"
+                "title": "ML Systems Design Grammar Contributors",
+                "marker": "DESIGNGRAMMAR"
             }
         }
     ],