rebar.config.sample: fix erl_opts example

rebar.config.sample

@@ -16,7 +16,9 @@
 {erl_first_files, ["mymib1", "mymib2"]}.
 
 %% Erlang compiler options
-{erl_opts, [no_debug_info, {i, "myinclude"}, {src_dirs, ["src1", "src2"]},
+{erl_opts, [no_debug_info,
+            {i, "myinclude"},
+            {src_dirs, ["src", "src2", "src3"]},
             {platform_define,
              "(linux|solaris|freebsd|darwin)", 'HAVE_SENDFILE'},
             {platform_define, "(linux|freebsd)", 'BACKLOG', 128},