Improve code style in tutorial (#293)

Prefer square bracket access for columns since dotted access is only for convenience.
Shorten long lines for readability.

Author: wjandrea

docs/getting/tutorial.rst

@@ -26,12 +26,9 @@ Next, we create a DataFrame with PintArrays as columns.
 
 .. ipython:: python
 
-   df = pd.DataFrame(
-      {
-         "torque": pd.Series([1.0, 2.0, 2.0, 3.0], dtype="pint[lbf ft]"),
-         "angular_velocity": pd.Series([1.0, 2.0, 2.0, 3.0], dtype="pint[rpm]"),
-      }
-   )
+   df = pd.DataFrame({
+       "torque": pd.Series([1, 2, 2, 3], dtype="pint[lbf ft]")
+       "angular_velocity": pd.Series([1, 2, 2, 3], dtype="pint[rpm]")})
    df
 
 
@@ -61,19 +58,19 @@ Each column can be accessed as a Pandas Series
 
 .. ipython:: python
 
-   df.power
+   df["power"]
 
 Which contains a PintArray
 
 .. ipython:: python
 
-   df.power.values
+   df["power"].values
 
 The PintArray contains a Quantity
 
 .. ipython:: python
 
-   df.power.values.quantity
+   df["power"].values.quantity
 
 DataFrame Index
 -----------------------
@@ -93,8 +90,8 @@ Methods that return arrays will be converted to Series.
 
 .. ipython:: python
 
-   df.power.pint.units
-   df.power.pint.to("kW")
+   df["power"].pint.units
+   df["power"].pint.to("kW")
 
 
 That's the basics! More examples are given at :doc:`Reading from csv <../user/reading>`.