Update conventions.md

Author: poulad

src/cargo/conventions.md

@@ -12,7 +12,7 @@ foo
 Suppose that we wanted to have two binaries in the same project, though. What
 then?
 
-It turns out that `cargo` supports this. The default binary name is `main.rs`, as
+It turns out that `cargo` supports this. The default binary name is `main`, as
 we saw before, but you can add additional binaries by placing them in a `bin/`
 directory: