PKGBUILD.5: Add fossil to list of supported VCSs

Since commit 08f4ae70

, makepkg supports downloading from fossil.
However, the PKGBUILD man page was only partially updated to reflect
this change.

Signed-off-by: Allan McRae <allan@archlinux.org>

doc/PKGBUILD.5.asciidoc

@@ -476,9 +476,9 @@ control system (VCS) is enabled by specifying the source in the form:
 
 	source=('directory::url#fragment?query')
 
-Currently makepkg supports the Bazaar, Git, Subversion, and Mercurial version
-control systems. For other version control systems, manual cloning of upstream
-repositories must be done in the `prepare()` function.
+Currently makepkg supports the Bazaar, Git, Subversion, Fossil and Mercurial
+version control systems. For other version control systems, manual cloning of
+upstream repositories must be done in the `prepare()` function.
 
 The source URL is divided into four components: