Replace yarn run and yarn install with yarn, remove building with npm instructions (#1528)

* Replace `yarn run` with `yarn`

* Replace `yarn install` with `yarn`

* Remove building using `npm` from node.md

dockers/Screenshotter/README.md

@@ -8,7 +8,7 @@ installed and running.
 If all you want is (re)create
 all the snapshots for all the browsers, then you can do so by:
 
-    yarn run test:screenshots:update
+    yarn test:screenshots:update
 
 It will fetch all required selenium docker images, and use them to
 take screenshots.
@@ -69,7 +69,7 @@ Examples:
 
 You can verify screenshots by running:
 
-    yarn run test:screenshots
+    yarn test:screenshots
 
 or passing `--verify` option to `screenshotter.js` or `screenshotter.sh`.
 See above for more details.

dockers/texcmp/README.md

@@ -30,7 +30,7 @@ the even larger overhead of setting up docker and creating the initial
 image, then you may instead execute the commands
 
     cd dockers/texcmp
-    yarn install
+    yarn
     node texcmp.js
 
 from the root of your KaTeX directory tree.  Required tools include the