Fix 'service install' on Mac OS

[povilasb]

cloudflared service install is not working as documented at #327

When cloudflared is started with Mac OS launch agent it errors with:

Use `cloudflared tunnel run` to start tunnel {UUID}
Use `cloudflared tunnel run` to start tunnel {UUID}
Use `cloudflared tunnel run` to start tunnel {UUID}

A fix is suggested at #327 (comment)
And acknowledged by https://community.cloudflare.com/t/tunnel-error-on-macos-when-installed-as-service/339543/4

However, collective costs keep compounding.

The fix seems simple at least for my use case where only a single tunnel is used. But Not sure if there's other negative side effects.
cloudflared service install produces such .plist

cat ~/Library/LaunchAgents/com.cloudflare.cloudflared.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
        <dict>
                <key>Label</key>
                <string>com.cloudflare.cloudflared</string>
                <key>ProgramArguments</key>
                <array>
                        <string>cloudflared</string>
                        <string>tunnel</string>
                        <string>run</string>
                </array>
                <key>RunAtLoad</key>
                <true/>
                <key>StandardOutPath</key>
                <string>/Users/povilas/Library/Logs/com.cloudflare.cloudflared.out.log</string>
                <key>StandardErrorPath</key>
                <string>/Users/povilas/Library/Logs/com.cloudflare.cloudflared.err.log</string>
                <key>KeepAlive</key>
                <dict>
                        <key>SuccessfulExit</key>
                        <false/>
                </dict>
                <key>ThrottleInterval</key>
                <integer>5</integer>
        </dict>
</plist>

as opposed to

                <array>
                        <string>cloudflared</string>
                </array>