docs(guides/installation.md): make azure devops http basic authn username empty

reflect the documentation on azure devops about using personal access token for http authentication:
https://docs.microsoft.com/en-us/azure/devops/organizations/accounts/use-personal-access-tokens-to-authenticate?view=azure-devops&tabs=preview-page#use-a-pat

Signed-off-by: maxgio92 <massimiliano.giovagnoli.1992@gmail.com>

docs/guides/installation.md

@@ -358,7 +358,7 @@ If you don't specify the SSH algorithm, then `flux` will generate an RSA 2048 bi
       --git-implementation=libgit2 \
       --url=https://dev.azure.com/<org>/<project>/_git/<repository> \
       --branch=master \
-      --username=git \
+      --username='' \
       --password=token \
       --interval=1m
     ```