fix detection of erlang files

While I was diving through the sync source code, I noticed that any file that ends in `erl`, `hrl`, or `dtl` seems to get picked up by `sync_utils:wildcard`. Looks like a slight tweak to the regex in `sync_scanner` fixes this:

Without tweaked regex:
```erlang
filelib:fold_files(Dir, "\.erl", true, fun(Y, Acc1) -> [Y|Acc1] end, []).
["/Users/danieltrinh/src/oss/sentry/lib/test.herl",
 "/Users/danieltrinh/src/oss/sentry/lib/test.erl"]
```

With tweaked regex:
```erlang
filelib:fold_files(Dir, "\\.erl", true, fun(Y, Acc1) -> [Y|Acc1] end, []).
["/Users/danieltrinh/src/oss/sentry/lib/test.erl"]
```

I don't know if this actually impacts the end behavior of compiling / reloading erlang code, but it might be useful to merge this for people like me who want to use the actual library code for other applications.