HV-1577 Fix a quote that has been mistakenly dropped in the previous commit

Author: gsmet

README.md

@@ -79,7 +79,7 @@ There are more build options available as well. For more information refer to [C
 
 To build Hibernate Validator with JDK 9, export the following environment variable:
 
-    export MAVEN_OPTS="--add-opens=java.base/java.security=ALL-UNNAMED --add-opens=java.base/java.lang=ALL-UNNAMED
+    export MAVEN_OPTS="--add-opens=java.base/java.security=ALL-UNNAMED --add-opens=java.base/java.lang=ALL-UNNAMED"
 
 Then the build can be started like this: