Fix warnings over deprecated comment syntax

[joeljuca]

Hello,

I just got started with EventStore, and after installing it and compiling my project, I got the following warnings:

==> eventstore
Compiling 60 files (.ex)
    warning: <%# is deprecated, use <%!-- or add a space between <% and # instead
    │
  1 │ <%#
    │ ~
    │
    └─ lib/event_store/sql/statements/insert_events.sql.eex:1: (file)

    warning: <%# is deprecated, use <%!-- or add a space between <% and # instead
    │
 24 │   <%#
    │   ~
    │
    └─ lib/event_store/sql/statements/insert_events.sql.eex:24: (file)

It seemed like a very simple thing to solve – hence, this PR. :)

Thank you for building EventStore and Commanded!