Fixes repo branch endpoint summary (#4893)

in browser saw `/repos/{owner}/{repo}/branches/{branch} List a repository's branches` fixed

Addresses https://github.com/go-gitea/debian-packaging/pull/1

Fixes https://github.com/go-gitea/gitea#4892

routers/api/v1/repo/branch.go

 func GetBranch(ctx *context.APIContext) {
 	// swagger:operation GET /repos/{owner}/{repo}/branches/{branch} repository repoGetBranch
 	// ---
-	// summary: List a repository's branches
+	// summary: Retrieve a specific branch from a repository
 	// produces:
 	// - application/json
 	// parameters:

templates/swagger/v1_json.tmpl

         "tags": [
           "repository"
         ],
-        "summary": "List a repository's branches",
+        "summary": "Retrieve a specific branch from a repository",
         "operationId": "repoGetBranch",
         "parameters": [
           {