GHC-10498: fix grammar

Co-authored-by: Jan Hrcek <[email protected]>

Author: Bodigrim

message-index/messages/GHC-10498/index.md

@@ -6,7 +6,7 @@ introduced: 9.6.1
 ---
 
 GHC does not allow the same entity to have more than one
-definitions. In some contexts it can figure out which one
-supposed to shadow another: local definitions take
+definition. In some contexts it can figure out which one
+is supposed to shadow another: local definitions take
 priority over global definitions, etc. Shadowing is just a warning, GHC-63397. However, if the definitions are on the same
 level it becomes a fatal error.