docs: fix grammar and capitalize word (#557)

Author: mikemcbride

docs/guide/markdown.md

@@ -239,7 +239,7 @@ You can import code snippets from existing files via following syntax:
 <<< @/test/markdown/fragments/snippet.js{2}
 
 ::: tip
-  Since the import of the code snippets will be executed before webpack compilation, so you can't use the path alias in webpack. the default value of `@` is `process.cwd()`.
+  Since the import of the code snippets will be executed before webpack compilation, you can't use the path alias in webpack. The default value of `@` is `process.cwd()`.
 :::