Add imports to example in README (#1796)

README.md

@@ -28,6 +28,7 @@ Polars is a blazingly fast DataFrames library implemented in Rust using Apache A
 To learn more, read the [User Guide](https://pola-rs.github.io/polars-book/).
 
 ```python
+>>> import polars as pl
 >>> df = pl.DataFrame(
     {
         "A": [1, 2, 3, 4, 5],
@@ -44,12 +45,12 @@ To learn more, read the [User Guide](https://pola-rs.github.io/polars-book/).
     .select([
     "fruits",
     "cars",
-    lit("fruits").alias("literal_string_fruits"),
-    col("B").filter(col("cars") == "beetle").sum(),
-    col("A").filter(col("B") > 2).sum().over("cars").alias("sum_A_by_cars"),       # groups by "cars"
-    col("A").sum().over("fruits").alias("sum_A_by_fruits"),                        # groups by "fruits"
-    col("A").reverse().over("fruits").flatten().alias("rev_A_by_fruits"),          # groups by "fruits
-    col("A").sort_by("B").over("fruits").flatten().alias("sort_A_by_B_by_fruits")  # groups by "fruits"
+    pl.lit("fruits").alias("literal_string_fruits"),
+    pl.col("B").filter(col("cars") == "beetle").sum(),
+    pl.col("A").filter(col("B") > 2).sum().over("cars").alias("sum_A_by_cars"),       # groups by "cars"
+    pl.col("A").sum().over("fruits").alias("sum_A_by_fruits"),                        # groups by "fruits"
+    pl.col("A").reverse().over("fruits").flatten().alias("rev_A_by_fruits"),          # groups by "fruits
+    pl.col("A").sort_by("B").over("fruits").flatten().alias("sort_A_by_B_by_fruits")  # groups by "fruits"
 ]))
 shape: (5, 8)
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