private/check-rst: quote more.

Author: jwilk

private/check-rst

@@ -38,7 +38,7 @@ then
 fi
 if [ $# -eq 0 ]
 then
-    grep -rwl ft=rst "${root}doc/"
+    grep -rwl 'ft=rst' "${root}doc/"
 else
     printf '%s\n' "$@"
 fi |