resources: update square README

Update square README to include a header for the pre-built binary.
Other applications use this format, so this update helps ensure
a more consistent template.

Change-Id: Iec273a7c1e2426e269eac1997668d6737a85dad1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-resources/+/50950
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/src/gpu/square/README.md b/src/gpu/square/README.md
index 26ec381..3ca07e3 100644
--- a/src/gpu/square/README.md
+++ b/src/gpu/square/README.md
@@ -23,6 +23,8 @@
 
 The compiled binary can be found in the `bin` directory.
 
+## Pre-built binary
+
 A pre-built binary can be found at <http://dist.gem5.org/dist/v21-1/test-progs/square/square>.
 
 ## Compiling GCN3_X86/gem5.opt