HADOOP-15706. Typo in compatibility doc: SHOUD -> SHOULD

(Contributed by Laszlo Kollar via Daniel Templeton)

Change-Id: I6e2459d0700df7f3bad4eac8297a11690191c3ba

hadoop-common-project/hadoop-common/src/site/markdown/Compatibility.md

@@ -187,7 +187,7 @@ existing documentation and tests and/or adding new documentation or tests.
 
 #### Java Binary compatibility for end-user applications i.e. Apache Hadoop ABI
 
-Apache Hadoop revisions SHOUD retain binary compatability such that end-user
+Apache Hadoop revisions SHOULD retain binary compatability such that end-user
 applications continue to work without any modifications. Minor Apache Hadoop
 revisions within the same major revision MUST retain compatibility such that
 existing MapReduce applications (e.g. end-user applications and projects such