Update README.md

Fix typo

README.md

@@ -1,7 +1,7 @@
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-# Adyen Client Slide Encryption Tester
+# Adyen Client Side Encryption Tester
 This Docker container allows you to generate the client side encrypted payload for Adyen using a simple web interface.
 
 With the generated payload, you can use it to authorise a payment through the Adyen API.