adding basic atbash problem

flip-my-letters/description.md

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+I dropped my alphabet on its head, can you help me reassemble it? `easyctf{r_wlmg_vevm_mvvw_zm_zhxrr_gzyov}`

flip-my-letters/grader.py

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+def grade(autogen, key):
+    if key.find("i_dont_even_need_an_ascii_table") != -1:
+        return True, "Correct!"
+    return False, "Nope!"

flip-my-letters/problem.yml

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+author: GenericNickname
+title: Flip My Letters
+hint: What happens if you turn the alphabet upside down?
+category: Cyrptography
+autogen: false
+programming: false
+value: 20
+files: