RegexBlock error_message typo in docs

In the documention an example had a singular

CONTRIBUTORS.rst

@@ -201,6 +201,7 @@ Contributors
 * John Heasly
 * Nikolai Røed Kristiansen
 * Alex Zagorodniuk
+* glassresistor
 
 Translators
 ===========

docs/topics/streamfield.rst

@@ -132,7 +132,7 @@ A single-line text input that validates a string against a regex expression. The
 
 .. code-block:: python
 
-    blocks.RegexBlock(regex=r'^[0-9]{3}$', error_message={
+    blocks.RegexBlock(regex=r'^[0-9]{3}$', error_messages={
         'invalid': "Not a valid library card number."
     })