Update TESTNAME description

Author: rthomas

CONTRIBUTING.md

@@ -131,7 +131,7 @@ Some common make targets are:
   & everything builds in the correct manner.
 - `make check-stage1-std NO_REBUILD=1` - test the standard library without
   rebuilding the entire compiler
-- `make check TESTNAME=<substring-of-test-name>` - Run a single test file
+- `make check TESTNAME=<substring-of-test-name>` - Run a matching set of tests.
   - `TESTNAME` should be a substring of the tests to match against e.g. it could 
     be the fully qualified test name, or just a part of it. 
     `TESTNAME=collections::hash::map::test_map::test_capacity_not_less_than_len`