Use append in code snippet

Fix code snippet that incorrectly truncated `bundler.toml`

Signed-off-by: Aidan Delaney <[email protected]>

Author: AidanDelaney

content/docs/buildpack-author-guide/create-buildpack/caching.md

@@ -169,7 +169,7 @@ else
     # Determine if there has been a gem dependency change and install new gems to the bundler layer; re-using existing and un-changed gems
     echo "---> Installing gems"
     mkdir -p "$bundlerlayer"
-    cat > "$layersdir/bundler.toml" << EOL
+    cat >> "$layersdir/bundler.toml" << EOL
 [metadata]
 checksum = "$local_bundler_checksum"
 EOL