add more precision to the effects of --bin flag (#1607)

`cargo build` will include *all* binaries by default, so it is misleading to say
"default or other binaries".

By using the the word `only` as an adverb, we implicitly communicating this
while also explaining the side effects of `--bin` correctly.

src/cargo/conventions.md

@@ -25,9 +25,9 @@ foo
         └── my_other_bin.rs
 ```
 
-To tell `cargo` to compile or run this binary as opposed to the default or other
-binaries, we just pass `cargo` the `--bin my_other_bin` flag, where `my_other_bin`
-is the name of the binary we want to work with.
+To tell `cargo` to only compile or run this binary, we just pass `cargo` the
+`--bin my_other_bin` flag, where `my_other_bin` is the name of the binary we
+want to work with.
 
 In addition to extra binaries, `cargo` supports [more features] such as
 benchmarks, tests, and examples.