[man] Fix typo in environment variable name

The section was added in 410a200ecd07f5e53ffea858d204b3bbf88d311d but with the wrong name.

man/mono.1

@@ -1100,7 +1100,7 @@ a Mono process through the environment.   This is useful for example
 to force all of your Mono processes to use LLVM or SGEN without having
 to modify any launch scripts.
 .TP
-\fBMONO_ENV_OPTIONS\fR
+\fBMONO_SDB_ENV_OPTIONS\fR
 Used to pass extra options to the debugger agent in the runtime, as they were passed
 using --debugger-agent=.
 .TP