caucase.storage: Fix docstring typo.

caucase/storage.py

@@ -315,7 +315,7 @@ class SQLite3Storage(local):
     override_limits=False,
   ):
     """
-    Store acertificate signing request and generate a unique ID for it.
+    Store a certificate signing request and generate a unique ID for it.
     Note: ID uniqueness is only guaranteed among pending CSR, and may be reused
     after the original CSR has been discarded (by being rejected or signed).
     """