Fix creyD#78

Fix creyD#78 by updating the regex.

I tested the regex but was not sure how to test the action. The regex should cover all three valid cases outlined here https://prettier.io/docs/en/plugins.html.

entrypoint.sh

@@ -59,7 +59,7 @@ if [ -n "$INPUT_PRETTIER_PLUGINS" ]; then
     for plugin in $INPUT_PRETTIER_PLUGINS; do
         echo "Checking plugin: $plugin"
         # check regex against @prettier/xyz
-        if ! echo "$plugin" | grep -Eq '(@prettier\/)+(plugin-[a-z\-]+)'; then
+        if ! echo "$plugin" | grep -Eq '(@prettier\/plugin-|(@[a-z\-]+\/)?prettier-plugin-){1}([a-z\-]+)'; then
             echo "$plugin does not seem to be a valid @prettier/plugin-x plugin. Exiting."
             exit 1
         fi