Update documentation on build space from sp to bd

It was decided that sp is too close to single port, and should be renamed bd to reference the build space.

Closes #338
diff --git a/docs/contents/libraries.rst b/docs/contents/libraries.rst
index ca3c24e..b7b29de 100644
--- a/docs/contents/libraries.rst
+++ b/docs/contents/libraries.rst
@@ -34,7 +34,7 @@
   +--------------------------------+---------------------------------------+
   | Digital Standard Cells         | :lib_type:`sc`                        |
   +--------------------------------+---------------------------------------+
-  | Build Space (Flash, SRAM, etc) | :lib_type:`sp`                        |
+  | Build Space (Flash, SRAM, etc) | :lib_type:`bd`                        |
   +--------------------------------+---------------------------------------+
   | IO and Periphery               | :lib_type:`io`                        |
   +--------------------------------+---------------------------------------+