Correct schema migration instructions for MySQL in 2.12.1 release notes

For MySQL the column type should be 'text', not 'clob'.

Change-Id: I22a99c8c92b3218eecb4a203d4106176fcf2a5c8

ReleaseNotes/ReleaseNotes-2.12.1.txt

diff --git a/ReleaseNotes/ReleaseNotes-2.12.1.txt b/ReleaseNotes/ReleaseNotes-2.12.1.txt
index cd1a304..f49de7d 100644
--- a/ReleaseNotes/ReleaseNotes-2.12.1.txt
+++ b/ReleaseNotes/ReleaseNotes-2.12.1.txt
@@ -21,12 +21,18 @@
 table must be manually migrated using the `gerrit gsql` SSH command or the
 `gqsl` site program.
 
-For the default H2 database or MySQL, execute the command:
+For the default H2 database, execute the command:
 
 ----
   alter table patch_sets modify push_certficate clob;
 ----
 
+For MySQL, execute the command:
+
+----
+  alter table patch_sets modify push_certficate text;
+----
+
 For PostgreSQL, execute the command:
 
 ----