configure.in: fix typo

* configure.in: remove extra right-bracket.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47876 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

configure.in

@@ -4030,7 +4030,7 @@ if test "${universal_binary-no}" = yes ; then
 else
     arch="${target_cpu}-${target_os}"
     AS_CASE(["$arch"], [le32-nacl], [arch="pnacl"])
-    AC_DEFINE_UNQUOTED(RUBY_PLATFORM, "$arch")]
+    AC_DEFINE_UNQUOTED(RUBY_PLATFORM, "$arch")
 fi
 
 unset sitearch