fix: make check-bbr (#4004)

## Overview

very minor fix that fixes the `check-bbr` command

(cherry picked from commit c410d88)

Makefile

@@ -251,10 +251,10 @@ prebuilt-binary:
 ## check-bbr: Check if your system uses BBR congestion control algorithm. Only works on Linux.
 check-bbr:
 	@echo "Checking if BBR is enabled..."
-	@if [ "$(sysctl net.ipv4.tcp_congestion_control | awk '{print $3}')" != "bbr" ]; then \
-	    echo "WARNING: BBR is not enabled. Please enable BBR for optimal performance. Call make enable-bbr or see Usage section in the README."; \
+	@if [ "$$(sysctl net.ipv4.tcp_congestion_control | awk '{print $$3}')" != "bbr" ]; then \
+		echo "WARNING: BBR is not enabled. Please enable BBR for optimal performance. Call make enable-bbr or see Usage section in the README."; \
 	else \
-	    echo "BBR is enabled."; \
+		echo "BBR is enabled."; \
 	fi
 .PHONY: check-bbr