Change to single quote warning/error messages

Signed-off-by: Alan Cha <[email protected]>

Author: Alan-Cha

packages/oasgraph-cli/src/oasgraph.ts

@@ -97,7 +97,7 @@ Promise.all(
   })
   .catch(filePath => {
     console.error(
-      `OASGraph cannot read file. File "${filePath}" does not exist.`
+      `OASGraph cannot read file. File '${filePath}' does not exist.`
     )
     process.exit(1)
   })