corrected typo

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

docs/defines.txt

@@ -1,5 +1,5 @@
 Feeble attempt to document the horde of #defines we deal with.
-Editors, plese make your descriptions short but informative.
+Editors, please make your descriptions short but informative.