README: make paragraphs before example code end with "::" for reST syntax

After this patch, README as reST text can be rendered into HTML more
appropriately. This is useful for hosting servers, which render README
file as a part of index HTML of its directory (for example,
Bitbucket).

README

@@ -4,14 +4,14 b' Mercurial'
 Mercurial is a fast, easy to use, distributed revision control tool
 for software developers.
 
-Basic install:
+Basic install::
 
  $ make            # see install targets
  $ make install    # do a system-wide install
  $ hg debuginstall # sanity-check setup
  $ hg              # see help
 
-Running without installing:
+Running without installing::
 
  $ make local      # build for inplace usage
  $ ./hg --version  # should show the latest version