docs: fix a typo in Dockerfile.5.md

s/Exececutable/Executable

Signed-off-by: Chen Hanxiao <chenhanxiao@cn.fujitsu.com>

docs/man/Dockerfile.5.md

@@ -101,7 +101,7 @@ A Dockerfile is similar to a Makefile.
   -- **CMD** has three forms:
 
   ```
-  # Exececutable form
+  # Executable form
   CMD ["executable", "param1", "param2"]`
 
   # Provide default arguments to ENTRYPOINT