Fix typo in docstring.
diff --git a/Lib/decimal.py b/Lib/decimal.py
index 0e6a966..4d9dd86 100644
--- a/Lib/decimal.py
+++ b/Lib/decimal.py
@@ -5862,7 +5862,7 @@
 
 def _convert_for_comparison(self, other, equality_op=False):
     """Given a Decimal instance self and a Python object other, return
-    an pair (s, o) of Decimal instances such that "s op o" is
+    a pair (s, o) of Decimal instances such that "s op o" is
     equivalent to "self op other" for any of the 6 comparison
     operators "op".