[doc] Add an example for 'missing-format-string-key' (#8379)

Author: Pierre-Sassoulas

doc/data/messages/m/missing-format-string-key/bad.py

@@ -0,0 +1,7 @@
+# +1: [missing-format-string-key]
+fruit_prices = """
+Apple: %(apple_price)d ¤
+Orange: %(orange_price)d ¤
+""" % {
+    "apple_price": 42
+}

doc/data/messages/m/missing-format-string-key/details.rst

@@ -1 +1,7 @@
-# This is a placeholder for correct code for this message.
+fruit_prices = """
+Apple: %(apple_price)d ¤
+Orange: %(orange_price)d ¤
+""" % {
+    "apple_price": 42,
+    "orange_price": 87,
+}