Fix github issue template

Refer to `docker-app` as a plugin (`docker app`)

Signed-off-by: Ulysses Souza <ulysses.souza@docker.com>

Author: Ulysses Souza

.github/ISSUE_TEMPLATE.md

@@ -55,7 +55,7 @@ Briefly describe the problem you are having in a few paragraphs.
 (paste your output here)
 ```
 
-**Output of `docker-app version`:**
+**Output of `docker app version`:**
 
 ```
 (paste your output here)