docs: fix markdown lint issues in command docs

cmd/gitannex/gitannex.go

@@ -539,7 +539,7 @@ var command = &cobra.Command{
 	Aliases: []string{uniqueCommandName},
 	Use:     subcommandName,
 	Short:   "Speaks with git-annex over stdin/stdout.",
-	Long:    gitannexHelp,
+	Long:    strings.TrimSpace(gitannexHelp),
 	Annotations: map[string]string{
 		"versionIntroduced": "v1.67.0",
 	},

cmd/gitannex/gitannex.md

@@ -4,8 +4,7 @@ users.
 
 [git-annex]: https://git-annex.branchable.com/
 
-Installation on Linux
----------------------
+### Installation on Linux
 
 1. Skip this step if your version of git-annex is [10.20240430] or newer.
    Otherwise, you must create a symlink somewhere on your PATH with a particular