Fix typo in test module usage message (GH-11374)

A minor typo in the output of `python -m test -h`.
A space was missing in between two words.
howmuch -> how much

Lib/test/libregrtest/cmdline.py

@@ -68,7 +68,7 @@ typically try to ascertain containers keep working when containing more than
 2 billion objects, which only works on 64-bit systems. There are also some
 tests that try to exhaust the address space of the process, which only makes
 sense on 32-bit systems with at least 2Gb of memory. The passed-in memlimit,
-which is a string in the form of '2.5Gb', determines howmuch memory the
+which is a string in the form of '2.5Gb', determines how much memory the
 tests will limit themselves to (but they may go slightly over.) The number
 shouldn't be more memory than the machine has (including swap memory). You
 should also keep in mind that swap memory is generally much, much slower