Documentation: fix git-prune example usage

Signed-off-by: Jonathon Mah <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jmah

Documentation/git-prune.txt

@@ -59,7 +59,7 @@ borrows from your repository via its
 `.git/objects/info/alternates`:
 
 ------------
-$ git prune $(cd ../another && $(git rev-parse --all))
+$ git prune $(cd ../another && git rev-parse --all)
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 Notes