docs(tutorial): fix GIT_SSH examples

They didn't show up as code-block

Related to #256

Author: Byron

doc/source/tutorial.rst

@@ -331,17 +331,19 @@ You can easily access configuration information for a remote by accessing option
     :start-after: # [26-test_references_and_objects]
     :end-before: # ![26-test_references_and_objects]
 
-You can also specify per-call custom environments using a new context manager on the Git command, e.g. for using a specific SSH key. The following example works with `git` starting at *v2.3*.
+You can also specify per-call custom environments using a new context manager on the Git command, e.g. for using a specific SSH key. The following example works with `git` starting at *v2.3*::
 
     ssh_cmd = 'ssh -i id_deployment_key'
     with repo.git.custom_environment(GIT_SSH_COMMAND=ssh_cmd):
         repo.remotes.origin.fetch()
 
-This one sets a custom script to be executed in place of `ssh`, and can be used in `git` prior to *v2.3*.
+This one sets a custom script to be executed in place of `ssh`, and can be used in `git` prior to *v2.3*::
 
     ssh_executable = os.path.join(rw_dir, 'my_ssh_executable.sh')
     with repo.git.custom_environment(GIT_SSH=ssh_executable):
         repo.remotes.origin.fetch()
+
+You might also have a look at `Git.update_environment(...)` in case you want to setup a changed environment more permanently.
 
 Submodule Handling
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