user-manual: fix missing colon in git-show example

There should be a colon in this git-show example.

Cc: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -1431,7 +1431,7 @@ modifying the working directory, you can do that with
 gitlink:git-show[1]:
 
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-$ git show HEAD^ path/to/file
+$ git show HEAD^:path/to/file
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 which will display the given version of the file.