docs: cURL write to a file instead of stdout (#25)

build/docs/content/05-merge.md

@@ -19,7 +19,7 @@ $ curl --request POST \
     --header 'Content-Type: multipart/form-data' \
     --form files=@file.pdf \
     --form files=@file2.pdf \
-    > result.pdf
+    -o result.pdf
 ```
 
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