connect: Boards use X and O, but they are called Black and White in description.md

[rbasso]

Both, description.md and canonical-data.json, use **X**s and **O**s to represent the pieces/marks in the boards, but in description.md, they are called Black and White.

This is unneedlessly confusing for users, specially if the test-suite is implemented using the colors.

We can solve this doing one of the following:

• Remove the references to colors in description.md.
• Change all **X**s and **O**s to **B**s and **W**s and fix the test descriptions.

description.md

The boards look like this (with spaces added for readability, which won't be in the representation passed to your code):

. O . X .
 . X X O .
  O O O X .
   . X O X O
    X O O O X

White (O) plays from top to bottom, black (X) plays from left to right. In
the above example white has made a connection from left to right but nobody has
won since white didn't connect top and bottom.

• The only references to color are those marked above.
• There are no references to colors in canonical-data.json or metadata.yml.