Update README.md - fix wrong variable name (#636)

new client.Counter should initiate counter variable and not gauge

README.md

@@ -356,19 +356,19 @@ Typescript can also enforce label names using `as const`
 ```typescript
 import * as client from 'prom-client';
 
-const gauge = new client.Counter({
+const counter = new client.Counter({
   name: 'metric_name',
   help: 'metric_help',
   // add `as const` here to enforce label names
   labelNames: ['method'] as const,
 });
 
 // Ok
-gauge.inc({ method: 1 });
+counter.inc({ method: 1 });
 
 // this is an error since `'methods'` is not a valid `labelName`
 // @ts-expect-error
-gauge.inc({ methods: 1 });
+counter.inc({ methods: 1 });
 ```
 
 #### Default Labels (segmented by registry)