decoder: Fix missing space in println frames

[Urhengulas]

It seems that "past me" (#651) has introduced a regression, namely that there is no space in-between timestamp and log messages for println frames.

Before (how it should be):

0 Hello, world!
└─ hello::__cortex_m_rt_main @ src/bin/hello.rs:8

After (how it currently is):

0Hello, world!
└─ hello::__cortex_m_rt_main @ src/bin/hello.rs:8

This PR fixes that.

[Urhengulas]

bors r+

[bors]

Build succeeded:

• ci