docs/manual: normalize delimiters of listing blocks

Although the asciidoc toolchain accepts any number of ~ to delimit a
listing block (i.e. a code block), it is actually specified to be
exactly four, i.e. ~~~~. Currently, a mix of diffrent numbers of ~ are
being used - sometimes even a different number at the beginning and at
the end of the block.

Normalize this to always use exactly four ~ for the delimiter.

Signed-off-by: Arnout Vandecappelle <arnout@mind.be>

docs/manual/adding-packages-tips.adoc

@@ -172,11 +172,11 @@ However, it is possible to download tarballs directly from the repository
 on GitHub. As GitHub is known to have changed download mechanisms in the
 past, the 'github' helper function should be used as shown below.
 
-------------------------
+----
 # Use a tag or a full commit ID
 FOO_VERSION = 1.0
 FOO_SITE = $(call github,<user>,<package>,v$(FOO_VERSION))
-------------------------
+----
 
 .Notes
 - The FOO_VERSION can either be a tag or a commit ID.
@@ -220,22 +220,22 @@ to download from Gitlab repositories. It can be used to download
 auto-generated tarballs produced by Gitlab, either for specific tags
 or commits:
 
-------------------------
+----
 # Use a tag or a full commit ID
 FOO_VERSION = 1.0
 FOO_SITE = $(call gitlab,<user>,<package>,v$(FOO_VERSION))
-------------------------
+----
 
 By default, it will use a +.tar.gz+ tarball, but Gitlab also provides
 +.tar.bz2+ tarballs, so by adding a +<pkg>_SOURCE+ variable, this
 +.tar.bz2+ tarball can be used:
 
-------------------------
+----
 # Use a tag or a full commit ID
 FOO_VERSION = 1.0
 FOO_SITE = $(call gitlab,<user>,<package>,v$(FOO_VERSION))
 FOO_SOURCE = foo-$(FOO_VERSION).tar.bz2
-------------------------
+----
 
 If there is a specific tarball uploaded by the upstream developers in
 +https://gitlab.com/<project>/releases/+, do not use this macro, but