MAPREDUCE-7389. Fix typo in description of property (#4440). Contributed by Christian Bartolomaus.

hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml

@@ -1722,7 +1722,7 @@
   must specify the appropriate classpath for that archive, and the name of
   the archive must be present in the classpath.
   If mapreduce.app-submission.cross-platform is false, platform-specific
-  environment vairable expansion syntax would be used to construct the default
+  environment variable expansion syntax would be used to construct the default
   CLASSPATH entries.
   For Linux:
   $HADOOP_MAPRED_HOME/share/hadoop/mapreduce/*,