updates jmeter ref to 5.3 on README; updates jmeter to 5.3 on run.sh as well

Author: rpherrera

README.md

@@ -69,7 +69,7 @@ Contribution by @wilsonmar
 
    The last line in the response should be:
 
-   <tt>Successfully tagged justb4/jmeter:5.1.1</tt>
+   <tt>Successfully tagged justb4/jmeter:5.3</tt>
 
 1. Run the test script:
 

run.sh

@@ -3,7 +3,7 @@
 # Run JMeter Docker image with options
 
 NAME="jmeter"
-IMAGE="justb4/jmeter:5.1.1"
+IMAGE="justb4/jmeter:5.3"
 
 # Finally run
 sudo docker stop ${NAME} > /dev/null 2>&1