Typo in description of "mapreduce.application.classpath" in mapred-default.xml

There is a small typo for

in the description of "mapreduce.application.classpath" in hadoop-mapreduce-project/hadoop-mapreduce-client/hadoop-mapreduce-client-core/src/main/resources/mapred-default.xml.

If mapreduce.app-submission.cross-platform is false, platform-specific
environment vairable expansion syntax would be used to construct the default
CLASSPATH entries.

I just stumbled upon that and will open a PR over at github.com.