runtime(doc): grammar fixes in doc (#13164)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/terminal.txt b/runtime/doc/terminal.txt
index c02a885..62bc6e6 100644
--- a/runtime/doc/terminal.txt
+++ b/runtime/doc/terminal.txt
@@ -1532,7 +1532,7 @@
 argument that is the name of the pty.
 							*gdb-version*
 Only debuggers fully compatible with gdb will work.  Vim uses the GDB/MI
-interface.  The "new-ui" command  requires gdb version 7.12 or later.  if you
+interface.  The "new-ui" command requires gdb version 7.12 or later.  If you
 get this error:
 	Undefined command: "new-ui". Try "help".~
 Then your gdb is too old.