Typescript Guide fix wording

Change "en variable" to "env variable" 
The standard for shorthand for an environment variable is "env" not "en."
Making this change will help improve understanding of what is meant here.

docs/guides/typescript.md

@@ -234,7 +234,7 @@ Under the hood, Hardhat uses [ts-node](https://www.npmjs.com/package/ts-node) to
 will recompile and type-check everything on every run. Depending on your project's size, this can get slow.
 
 You can make Hardhat run faster by preventing `ts-node` from type-checking your project. This is done by setting the
-`TS_NODE_TRANSPILE_ONLY` en variable to `1`. 
+`TS_NODE_TRANSPILE_ONLY` env variable to `1`. 
 
 For example, you can run your TypeScript-based tests faster like this `TS_NODE_TRANSPILE_ONLY=1 npx hardhat test`.