Update README.md

Author: Zeioth

README.md

@@ -58,7 +58,6 @@ On this example, we are loading all available external sources.
       'none-ls-external-sources.diagnostics.flake8',
       'none-ls-external-sources.diagnostics.luacheck',
       'none-ls-external-sources.diagnostics.psalm',
-      'none-ls-external-sources.diagnostics.shellcheck',
       'none-ls-external-sources.diagnostics.yamllint',
 
       -- formatting