Typo fix (#1388)

I'm torn between "Run this bash script to create this file" and "The
file should look like this" and open to ideas for clarity, but I think
adding the backslash is more likely to be noticed by someone who creates
the file without using the heredoc than the other way around.

Fixes #1335

docs/src/run-with-lambda.md

@@ -60,7 +60,7 @@ Every zip-based Lambda function runs a file called `bootstrap`.
 cat <<EOF >src/bootstrap
 #!/bin/sh
 set -eu
-exec martin --config ${_HANDLER}.yaml
+exec martin --config \${_HANDLER}.yaml
 EOF
 ```