Fix typo preventing the use of KUBE_PROXY_URL env variable (#2485)

* Fix typo preventing the use of KUBE_PROXY_URL provider environment variable

Author: alexsomesan

.changelog/2485.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+`kubernetes_manifest`: fix issue preventing KUBE_PROXY_URL environment variable from being used in client configuration (#1733)
+```

manifest/provider/configure.go

@@ -523,7 +523,7 @@ func (s *RawProviderServer) ConfigureProvider(ctx context.Context, req *tfprotov
 		}
 		overrides.ClusterDefaults.ProxyURL = proxyURL
 	}
-	if proxyUrl, ok := os.LookupEnv("KUBE_PROXY_URL"); ok && proxyUrl != "" {
+	if proxyURL, ok := os.LookupEnv("KUBE_PROXY_URL"); ok && proxyURL != "" {
 		overrides.ClusterDefaults.ProxyURL = proxyURL
 	}