fix escaping in message

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1903314 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/openxml4j/opc/PackagePartName.java

@@ -326,7 +326,7 @@ public final class PackagePartName implements Comparable<PackagePartName> {
             /* Check rule M1.7 */
             if (decodedChar == '/' || decodedChar == '\\') {
                 throw new InvalidFormatException(
-                    "A segment shall not contain percent-encoded forward slash ('/'), or backward slash ('\') characters. [M1.7]");
+                    "A segment shall not contain percent-encoded forward slash ('/'), or backward slash ('\\') characters. [M1.7]");
             }
 
             /* Check rule M1.8 */