Update formatting for the API docs

Author: ilikecake

docs/api.rst

@@ -1,4 +1,5 @@
-
+PCA9555
+------------
 .. If you created a package, create one automodule per module in the package.
 
 .. If your library file(s) are nested in a directory (e.g. /adafruit_foo/foo.py)
@@ -9,21 +10,33 @@
     :inherited-members:
     :members:
 
+PCAL9555
+------------
+
 .. automodule:: i2c_expanders.PCAL9555
     :show-inheritance:
     :inherited-members:
     :members:
 
+PCA9554
+------------
+
 .. automodule:: i2c_expanders.PCA9554
     :show-inheritance:
     :inherited-members:
     :members:
 
+PCAL9554
+------------
+
 .. automodule:: i2c_expanders.PCAL9554
     :show-inheritance:
     :inherited-members:
     :members:
 
+Common
+------------
+
 .. automodule:: i2c_expanders.i2c_expander
     :members: