Update build.mill

Author: lihaoyi

example/fundamentals/out-dir/1-out-files/build.mill

@@ -226,7 +226,7 @@ out/mill-server
 // Again, if there are multiple paths through which one task was invalidated by another,
 // one path is chosen arbitrarily to be shown in the spanning tree
 //
-// == `methodCodeHashSignatures spanningInvalidationTree`
+// === `methodCodeHashSignatures spanningInvalidationTree`
 //
 // Sometimes invalidation can be caused by a code change in your `build.mill`/`package.mill`
 // files, rather than by a change in the project's source files or inputs. In such cases,