docs: update help.txt (#1401)

Small update so that `nodemon -h` mentions that `.json` files are now watched by default (this is the case since v1.7.0 -  #643)

P.S.: one remark: the change fixing #643 may have been considered as a breaking change, and reflected by bumping major version number. Updating some apps to nodemon v1.7.0+ require adaptation of the config...
P.P.S. Thanks for creating nodemon, super useful piece of software.

doc/cli/help.txt

@@ -13,7 +13,7 @@
 
   Note: if the script is omitted, nodemon will try to read "main" from
   package.json and without a nodemon.json, nodemon will monitor .js, .mjs, .coffee,
-  and .litcoffee by default.
+  .litcoffee, and .json by default.
 
   For advanced nodemon configuration use nodemon.json: nodemon --help config
   See also the sample: https://github.com/remy/nodemon/wiki/Sample-nodemon.json