Fix read-only cmdline in credentials.asc

Author: toolness

book/07-git-tools/sections/credentials.asc

@@ -193,7 +193,7 @@ Since its name starts with ``git-'', we can use the simple syntax for the config
 
 [source,console]
 ----
-$ git config --global credential.helper read-only --file /mnt/shared/creds
+$ git config --global credential.helper 'read-only --file /mnt/shared/creds'
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 As you can see, extending this system is pretty straightforward, and can solve some common problems for you and your team.