prompt: Fix git submodule path for 'pure' with actual path

WARNING: This will require synchronizing submodules' remote URL
configuration setting to the value specified in `.gitmodules` by doing:
`git submodule sync --recursive` in your Prezto location.

Signed-off-by: Indrajit Raychaudhuri <irc@indrajit.com>

.gitmodules

@@ -16,7 +16,7 @@
 [submodule "modules/prompt/external/agnoster"]
 	path = modules/prompt/external/agnoster
 	url = https://github.com/agnoster/agnoster-zsh-theme.git
-[submodule "modules/prompt/functions/pure"]
+[submodule "modules/prompt/external/pure"]
 	path = modules/prompt/external/pure
 	url = https://github.com/sindresorhus/pure.git
 [submodule "modules/fasd/external"]