Update README.md: update the 'more details' url in the section 'TLS endpoint' (#2814)

* Update README.md: correct the wrong url(link to exporter-toolkit web-config) in the section 'TLS endpoint'

Signed-off-by: yang-stressfree <68363665+yang-stressfree@users.noreply.github.com>

* Update README.md

Co-authored-by: Ben Kochie <superq@gmail.com>
Signed-off-by: yang-stressfree <68363665+yang-stressfree@users.noreply.github.com>

---------

Signed-off-by: yang-stressfree <68363665+yang-stressfree@users.noreply.github.com>
Co-authored-by: Ben Kochie <superq@gmail.com>

README.md

@@ -377,7 +377,7 @@ The exporter supports TLS via a new web configuration file.
 ./node_exporter --web.config.file=web-config.yml
 ```
 
-See the [exporter-toolkit https package](https://github.com/prometheus/exporter-toolkit/blob/v0.1.0/https/README.md) for more details.
+See the [exporter-toolkit web-configuration](https://github.com/prometheus/exporter-toolkit/blob/master/docs/web-configuration.md) for more details.
 
 [travis]: https://travis-ci.org/prometheus/node_exporter
 [hub]: https://hub.docker.com/r/prom/node-exporter/