docs: use bash "type -p" because dash isn't useful

If /bin/sh is bash, the 'type' builtin produces a list of filenames
that match the arguments to $PATH.

If /bin/sh is dash, we get errors like:

	/bin/sh: 1: P:: not found

Hopefully having a build-dep on bash is not controversial.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

docs/Makefile

@@ -1,4 +1,4 @@
-MARKDOWN := $(firstword $(shell type -P markdown markdown2 markdown_py 2>/dev/null || echo markdown))
+MARKDOWN := $(firstword $(shell bash -c 'type -P markdown markdown2 markdown_py' 2>/dev/null || echo markdown))
 .PHONY: docs
 
 docs: $(subst .md,.html,$(wildcard *.md)) index.html ../README.html