Fix cmdline in credentials.asc

Author: toolness

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

@@ -36,7 +36,7 @@ Here's an example of how you'd configure the ``store'' helper with a custom file
 
 [source,console]
 ----
-$ git config --global credential.helper store --file ~/.my-credentials
+$ git config --global credential.helper 'store --file ~/.my-credentials'
 ----
 
 Git even allows you to configure several helpers.