x509_signed_commits: Make 'gpgsm' command copy-pastable

The syntax for Bash variable assignment is 'var=value', without any
spaces.  Make the 'gpgsm' command used to define the 'signingkey'
variable copy-pastable by removing the spaces on each side of the equal
sign.

doc/user/project/repository/x509_signed_commits/index.md

@@ -88,7 +88,7 @@ If you have the correct version, you can proceed to configure Git.
 Configure Git to use your key for signing:
 
 ```shell
-signingkey = $( gpgsm --list-secret-keys | egrep '(key usage|ID)' | grep -B 1 digitalSignature | awk '/ID/ {print $2}' )
+signingkey=$( gpgsm --list-secret-keys | egrep '(key usage|ID)' | grep -B 1 digitalSignature | awk '/ID/ {print $2}' )
 git config --global user.signingkey $signingkey
 git config --global gpg.format x509
 ```