Fix misc markdown lint errors

README.md

@@ -49,16 +49,16 @@ You can also build from sources by cloning this repo and running `sh build.sh`,
 
 ### ![fedora](https://user-images.githubusercontent.com/1690934/106691082-ce11bc00-65ca-11eb-8af5-92253e6ff7a2.png) Fedora
 
-htmltest is in the [Fedora repositories](https://src.fedoraproject.org/rpms/htmltest), note this is not a first party build and may lag the latest version. 
+htmltest is in the [Fedora repositories](https://src.fedoraproject.org/rpms/htmltest), note this is not a first party build and may lag the latest version.
 
 ```dnf install htmltest```
 
 ### :whale: Docker
 
-```docker run -v $(pwd):/test --rm wjdp/htmltest```  
+```docker run -v $(pwd):/test --rm wjdp/htmltest```
 Mount your directory with html files into the container and test them.
 
-If you need more arguments to the test run it like this:  
+If you need more arguments to the test run it like this:
 ```docker run -v $(pwd):/test --rm wjdp/htmltest -l 3 -s```
 
 ### Notes
@@ -67,7 +67,7 @@ We store temporary files in `tmp/.htmltest` by default. You probably want to ign
 
 ## :computer: Usage
 
-```
+```txt
 htmltest - Test generated HTML for problems
            https://github.com/wjdp/htmltest