Fix 5_1 and 5_3

Exercises/ex5_1.md

@@ -115,7 +115,7 @@ kinds of input sources.  For example:
 ```python
 >>> import gzip
 >>> import stock
->>> file = gzip.open('Data/portfolio.csv.gz')
+>>> file = gzip.open('Data/portfolio.csv.gz', 'rt')
 >>> port = reader.csv_as_instances(file, stock.Stock)
 >>> port
 [Stock('AA', 100, 32.2), Stock('IBM', 50, 91.1), Stock('CAT', 150, 83.44), 

Exercises/soln5_3.md

@@ -48,11 +48,10 @@ def read_csv_as_instances(filename, cls, *, headers=None):
 import csv
 
 def convert_csv(lines, converter, *, headers=None):
-    records = []
     rows = csv.reader(lines)
     if headers is None:
         headers = next(rows)
-    return map(lambda row: converter(headers, row), rows)
+    return list(map(lambda row: converter(headers, row), rows))
 ```