Fix user.signingkey example

Since the value of user.signingkey is passed unchanged to gpg's
--local-user parameter, the fingerprint needs to be suffixed with an
exclamation mark (!).

If a fingerprint is specified without an exclamation mark, it is simply
ignored.

Author: kariya-mitsuru

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

@@ -29,7 +29,7 @@ Once you have a private key to sign with, you can configure Git to use it for si
 
 [source,console]
 ----
-$ git config --global user.signingkey 0A46826A
+$ git config --global user.signingkey 0A46826A!
 ----
 
 Now Git will use your key by default to sign tags and commits if you want.