docs: Fix npm-config reference to userconfig & globalconfig env vars

Credit: @racztiborzoltan
Reviewed-By: @kenany
Reviewed-By: @iarna
PR-URL: npm/npm#17369

doc/misc/npm-config.md

@@ -31,9 +31,9 @@ The four relevant files are:
 
 * per-project configuration file (`/path/to/my/project/.npmrc`)
 * per-user configuration file (defaults to `$HOME/.npmrc`; configurable via CLI
-  option `--userconfig` or environment variable `$NPM_CONF_USERCONFIG`)
+  option `--userconfig` or environment variable `$NPM_CONFIG_USERCONFIG`)
 * global configuration file (defaults to `$PREFIX/etc/npmrc`; configurable via
-  CLI option `--globalconfig` or environment variable `$NPM_CONF_GLOBALCONFIG`)
+  CLI option `--globalconfig` or environment variable `$NPM_CONFIG_GLOBALCONFIG`)
 * npm's built-in configuration file (`/path/to/npm/npmrc`)
 
 See npmrc(5) for more details.