CLEANUP: Fix a typo in fix_is_valid description

MsgType tag was misspelled.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index da108ab..cb4cd44 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -15630,7 +15630,7 @@
     numeric
   - checks the BeginString tag is the first tag with a valid FIX version
   - checks the BodyLength tag is the second one with the right body length
-  - checks the MstType tag is the third tag.
+  - checks the MsgType tag is the third tag.
   - checks that last tag in the message is the CheckSum tag with a valid
     checksum