migrations: fix missing comma in 000007

Change-Id: I489a8d3a2c999e7b3467a7442bff71d323424575
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/711162
Reviewed-by: Jonathan Amsterdam <jba@google.com>

migrations/000007_add_readme_package_imports_documentation_tables.up.sql

@@ -17,7 +17,7 @@ CREATE TABLE documentation (
     goos text NOT NULL,
     goarch text NOT NULL,
     synopsis text NOT NULL,
-    html text NOT NULL
+    html text NOT NULL,
     PRIMARY KEY (path_id, goos, goarch)
 );
 COMMENT ON TABLE documentation IS