Update README.md - Using ninja test command

I believe the intended command was `ninja`.
`meson -C ...` gives:
```
meson -C builddir test
usage: meson [-h] {setup,configure,dist,install,introspect,init,test,wrap,subprojects,rewrite,compile,devenv,env2mfile,help} ...
meson: error: unrecognized arguments: -C
```

Whereas `ninja -C builddir test` gives:
```
ninja: Entering directory `builddir'
[0/1] Running all tests.
1/6 piper / validate appdata file        OK              0.02s
2/6 piper / python-ruff-check            SKIP            0.01s   exit status 77
3/6 piper / svg-lookup-check             OK              0.04s
4/6 piper:all / files-in-git             OK              0.08s
5/6 piper / check-svg                    OK              0.09s
6/6 piper / python-black-check           OK              0.32s

Ok:                 5   
Expected Fail:      0   
Fail:               0   
Unexpected Pass:    0   
Skipped:            1   
Timeout:            0   

Full log written to /home/ratbag/Workspace/piper/builddir/meson-logs/testlog.txt
```

README.md

@@ -86,7 +86,7 @@ You can check if your code passes tests before submitting changes using the
 following command:
 
 ```sh
-meson -C builddir test
+meson test -C builddir
 ```
 
 Source