fixup! Update some outdated documentation

Author: achaikou

python/segyio/trace.py

@@ -629,7 +629,7 @@ def __getitem__(self, i):
         Read a field in the first 5 headers:
 
         >>> [x[25] for x in header[:5]]
-        [1, 2, 3, 4]
+        [1, 2, 3, 4, 5]
 
         Read a field in every other header: