doc: fix grammar and phrasing in typescript.md

Fix grammatical issues and heading phrasing in `doc/api/typescript.md`:
- Correct subject-verb agreement ("files that contains" -> "files that
  contain").
- Add missing indefinite article ("result in an ... error").
- Change heading from "Paths aliases" to "Path aliases" to match
  standard TypeScript terminology.
- Clarify verb parallelism ("won't be transformed and will therefore
  produce an error").

Signed-off-by: Ravi Arnan <raviarnankeren@gmail.com>

Author: ravi-arnan

doc/api/typescript.md

@@ -76,7 +76,7 @@ changes:
     description: Type stripping is now stable.
 -->
 
-By default Node.js will execute TypeScript files that contains only
+By default Node.js will execute TypeScript files that contain only
 erasable TypeScript syntax.
 Node.js will replace TypeScript syntax with whitespace,
 and no type checking is performed.
@@ -164,7 +164,7 @@ namespace TypeOnly {
 }
 ```
 
-This will result in [`ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX`][] error:
+This will result in an [`ERR_UNSUPPORTED_TYPESCRIPT_SYNTAX`][] error:
 
 ```ts
 // This namespace is exporting a value
@@ -231,9 +231,9 @@ is highly discouraged.
 Stripping types inside `node_modules` is intended only
 for local development.
 
-### Paths aliases
+### Path aliases
 
-[`tsconfig` "paths"][] won't be transformed and therefore produce an error. The closest
+[`tsconfig` "paths"][] won't be transformed and will therefore produce an error. The closest
 feature available is [subpath imports][] with the limitation that they need to start
 with `#`.