Compiler warning in UnsafeInMemorySorter class

This is a small starter task to help new contributors practice the pull request and code review process.

When building Spark with Java 8, there is a compiler warning in the class UnsafeInMemorySorter:

[WARNING] .../core/src/main/java/org/apache/spark/util/collection/unsafe/sort/UnsafeInMemorySorter.java:284: warning: [static] 
static method should be qualified by type name, TaskMemoryManager, instead of
 by an expression

Spark should compile without these kinds of warnings.