Update read1 example in gdb/testsuite/README

Tom de Vries noticed that the recent changes to the testsuite's
configury required an update to the README.  This patch changes the
text to document the new reality.

2021-06-09  Tom Tromey  <tromey@adacore.com>

	* README (Example): Update read1 example.

gdb/testsuite/ChangeLog

@@ -1,3 +1,7 @@
+2021-06-09  Tom Tromey  <tromey@adacore.com>
+
+	* README (Example): Update read1 example.
+
 2021-06-09  Simon Marchi  <simon.marchi@polymtl.ca>
 
 	* lib/tuiterm.exp (Term) <_check_box>: Improve logging.

gdb/testsuite/README

@@ -347,9 +347,13 @@ return at most 1 byte.
 To enable this, either pass a non-empty value in the READ1 make
 variable, or use the check-read1 make target instead of check.
 
-Examples:
+Example:
 
 	make -j10 check-read1 TESTS="*/paginate-*.exp"
+
+If you've already built the read1 support code, either via a previous
+'check-read1' run, or by using "make read1", you can use:
+
 	make -j10 check READ1="1"
 
 Note: While the intention is to detect races and make otherwise passing tests