[javadoc] Fix typo in javadoc of Strings instances (#1406)

Author: sebastian-steiner

src/main/java/org/apache/commons/lang3/Strings.java

@@ -268,7 +268,7 @@ public int lastIndexOf(final CharSequence seq, final CharSequence searchSeq, fin
     public static final Strings CI = new CiStrings(true);
 
     /**
-     * The <strong>C</strong>ase-<strong>S</strong>nsensitive singleton instance.
+     * The <strong>C</strong>ase-<strong>S</strong>ensitive singleton instance.
      */
     public static final Strings CS = new CsStrings(true);