grammar: Make apply statements have block bodies

Signed-off-by: Will Hawkins <hawkinsw@obs.cr>

tree-sitter-p4/test/corpus/control.txt

@@ -24,9 +24,7 @@ control simple() {
     (action)
     (identifier)
     (parameters)
-    (statement
-     (blockStatement)
-    )
+    (blockStatement)
    )
    (table_declaration
     (table)
@@ -51,9 +49,7 @@ control simple() {
    )
    (apply_statement
     (apply)
-    (statement
-     (blockStatement)
-    )
+    (blockStatement)
    )
   )
  )

tree-sitter-p4/test/corpus/statements.txt

@@ -15,12 +15,10 @@ control T() {
    (parameters)
    (apply_statement
     (apply)
-    (statement
-     (blockStatement
-      (statements
-       (statement
-        (exit_statement)
-       )
+    (blockStatement
+     (statements
+      (statement
+       (exit_statement)
       )
      )
     )