Update HOWTO-port-an-exercise.md (#296)

Fix error in the command line

Author: rabestro

HOWTO-port-an-exercise.md

@@ -28,7 +28,7 @@ Here's a step-by-step procedure to port the **_foo-bar_** exercise:
     # enter the exercise directory
     cd "exercises/practice/$exercise"
     # delete the bash solution and metadata:
-    rm ${exercise//-/_}.sh .meta/{test.toml,version,example.sh}
+    rm ${exercise//-/_}.sh .meta/{tests.toml,version,example.sh}
     # rename the test file. Note the underscore changed: the AWK track uses kebab case.
     mv ${exercise//-/_}.bats test-$exercise.bats
     # edit `.meta/config.json`