fix : delete `!` in usage part and rewrite the description

subjects/swapargs/README.md

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+## swap-args
+
+### Instructions
+
+Write a program that takes two arguments and swaps them.
+- If the number of arguments is different from 2, print (`'\n'`).
+- The arguments must be separated by a withe space.
+- Print (`'\n'`) at the end of the program.
+
+### Usage
+
+```console
+$ go run . | cat -e
+$
+$ go run . 1 2 | cat -e
+2 1$
+$ go run . "World" "Hello" | cat -e
+Hello World$
+```