Typo corrections in concepts.md (#418)

src/docs/markdown/caddyfile/concepts.md

@@ -469,7 +469,7 @@ If your configuration relies on environment variables, you can use them in the C
 
 Environment variables in this form are substituted **before Caddyfile parsing begins**, so they can expand to empty values (i.e. `""`), partial tokens, complete tokens, or even multiple tokens and lines.
 
-For example, a environement variable `UPSTREAMS="app1:8080 app2:8080 app3:8080"` would expand to multiple [tokens](#tokens-and-quotes):
+For example, an environment variable `UPSTREAMS="app1:8080 app2:8080 app3:8080"` would expand to multiple [tokens](#tokens-and-quotes):
 
 ```caddy
 example.com {