Documentation: Fix sphinx configuration

Without the brackets, the string seems to be added as a list of
characters, and since there's no extension called 's', sphinx
bails out.

Change-Id: If0fc9c1a74f334b6154df3cb26836509de913567
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/44114
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/conf.py

@@ -48,7 +48,7 @@ try:
 except ImportError:
     print("Error: Please install sphinxcontrib.ditaa for ASCII art conversion\n")
 else:
-    extensions += 'sphinxcontrib.ditaa'
+    extensions += ['sphinxcontrib.ditaa']
 
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.